Add release helper script (#64)

This commit adds a release helper script, amongst other workflow improvements.

See `README.md` for instructions, and `_tools/release.sh` for the script itself.

This script:

1. sanity-checks the various version tags;
2. triggers a Plugin build;
3. flags the new version as pre-release;
4. tags the version in Git;
5. creates a local working copy of the SVN repo;
6. copies the new release to the local `trunk/` (whilst keeping the ‘Stable Tag’ field in `readme.txt` pointing to the previous stable version);
7. copies the commit message from the changelog in `readme.txt`; and
8. (if a flag is set) commits the changes to the remote `trunk/`.

Pushing out a new release must still be done manually, once `trunk/` is tested and working. To do so, check out a local copy of `trunk/` and:

1. update the ‘Stable Tag’ field in `trunk/readme.txt` to the new version;
2. update the ‘Version’ field in the comment header of `trunk/footnotes.php` to the new version;
3. remove the ‘p’ from the end of the ‘version’ tag in the `getInfo()` function at the bottom of `js/wsiwyg-editor.js`; 
4. copy a new tag for the release from `trunk/` (`svn cp trunk tags/<version number>`); and
5. commit your changes (`svn ci -m "Release version <version number>"`).

The WP Plugin Directory will automatically parse the ‘Stable Tag’ field in `trunk/readme.txt`, and inform users that a new version is available.

At various stages user input is required to validate information. This is not ready for automation with GitHub Actions, but is a useful step on the way — see [this piece](https://blog.danslimmon.com/2019/07/15/do-nothing-scripting-the-key-to-gradual-automation/) for more info.

Unless a `-c` flag is passed (e.g., by running `composer run release:commit`) no changes will take place on the remote SVN repo. If you want to test this out on a branch other than `main`, uncomment lines 31 & 52 of the script.

Version checking enforces the versioning rules stated [here](https://github.com/markcheret/footnotes/wiki/Versioning).

**NB: I have not tested the `-c` mode yet, as I wanted people will more familiarity with the SVN to have a look at it before I risked making any changes and blowing everything up.**

Co-authored-by: pewgeuges <73141620+pewgeuges@users.noreply.github.com>

readme.txt

@@ -4,10 +4,7 @@ Tags: footnote, footnotes, bibliography, formatting, notes, Post, posts, referen
 Requires at least: 3.9
 Tested up to: 5.7
 Requires PHP: 5.6
-Package Version: 2.5.11d2
-Version: 2.5.10
 Stable Tag: 2.5.10
-CAUTION: THE S. T. FIELD IS PARSED FOR RELEASE CONFIGURATION.
 License: GPLv3 or later
 License URI: https://www.gnu.org/licenses/gpl-3.0.html
 
@@ -81,14 +78,14 @@ Visit this swift write-up from a **footnotes** user by the name of **Southwest**
 == Changelog ==
 
 = 2.5.11 =
-- Adding: Documentation: Readme.txt: comment line below the 'Stable Tag' field to warn that this is (unexpectedly) parsed for release configuration.
-- Adding: Documentation: Readme.txt: informative 'Version' field pointing a tag folder in SVN for bugfix versions available ahead of the stable tag.
-- Adding: Documentation: Readme.txt: informative 'Package Version' field in sync with the package version currently hidden in the main PHP script header.
-- Bugfix: Dashboard: debug text input fields by disabling quotation mark escapement, thanks to @rumperuu code contribution during the codebase conformance overhaul.
+- Update: Documentation: Readme.txt: comment line below the 'Stable Tag' field to warn that this field is (unintuitively) parsed for release configuration.
+- Update: Documentation: Readme.txt: informative 'Version' field in sync with 'Version' in 'footnotes.php' for bugfix versions available ahead of the Stable Tag.
+- Update: Documentation: Readme.txt: informative 'Package Version' field in sync with the 'Package V.' field added in the 'footnotes.php' file header.
+- Bugfix: Dashboard: debug text input fields by disabling quotation mark escapement, thanks to @rumperuu code contribution in the standards compliance overhaul.
 - Update: Codebase: make all PHP code comply to WordPress Coding Standards requirements, thanks to @rumperuu code contribution and refactoring.
-- Adding: Documentation: additional readme in markdown format 'README.md' for use with GitHub, thanks to @rumperuu code contribution.
-- Adding: Documentation: help and support for contributors by the means of Contributing Guidelines 'CONTRIBUTING.md', thanks to @rumperuu code contribution.
-- Adding: Development: pre-commit hook for WordPress projects, modified from @bjornjohansen, thanks to @rumperuu code contribution.
+- Update: Documentation: help and support for contributors by the means of Contributing Guidelines in 'CONTRIBUTING.md', thanks to @rumperuu code contribution.
+- Update: Documentation: additional readme in markdown format 'README.md' for use with the code repository on GitHub, thanks to @rumperuu code contribution.
+- Update: Documentation: move 'customized-template-stack.txt' and 'customized-documentation-schema.txt' from the 'development/' folder to root.
 
 = 2.5.10 =
 - Bugfix: Codebase: revert to 2.5.8 with apologies (below), thanks to @little-shiva @watershare @adjayabdg @staho @frav8 @voregnev @dsl225 @alexclassroom @a223123131 @codldmac bug reports.