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>

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css/tmp/footnotes-nottbrpl3.min.css

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