ci: improve linting, GitHub Actions workflows (#149)

* release: release 2.7.2

* fix: urgent 2.7.3 release to fix fatal error

* chore: remove un-needed setup script

* ci: remove steps from pre-push command

* chore: remove un-needed PHP-Commitizen config

* chore: put image files in correct folders

* chore: move GitHub image into `.github/` dir

* fix: classic editor button

* fix: call correct jQuery Tools file in dev env

* docs: replace license with Markdown version

* ci: clean up PHP linting commands

If anyone has noticed me playing musical filepaths with these commands
for a while, it's because I kept getting inconsistent results from the
use of double-globs (i.e., `**`). However, I've finally figured out
that this is because Composer is running these scripts in its own shell,
so the double-glob that works when I run the command manually in my
Terminal doesn't work when Composer runs it in its.

* chore: lint PHP files

* build: update JS linting standards

* chore: lint JS file

* build: add node-sass

* build: add additional stylelint rules

* chore: lint stylesheets with new rulesets

* refactor: move ESLint settings to `package.json`

* chore: move Prettier config to `package.json`

It is not yet possible to move `.prettierignore` to `package.json` too,
but this appears to be on the horizon; see [this Issue][prettier-issue].

[prettier-issue]: https://github.com/prettier/prettier/issues/3460

* fix: move WPML config into Plugin folder

* chore: move Stylelint config into `package.json`

* chore: remove unused `.distignore`

It can always be re-added at a later date if it becomes useful.

* chore: format file

* build: add HTML linting

* fix: add image alt tag

* ci: clean up GitHub Actions workflows

* fix: fix workflow

* fix: fix indentation

* ci: add YAML validation

* chore: make valid

* ci: add YAML validation

* chore: lint code

* ci: change dep install back to original

* chore: lint license

.github/workflows/lint-code.yml

@@ -0,0 +1,49 @@
+name: Code Linting
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  lint:
+    name: Lint all code
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Validate composer.json and composer.lock
+        run: composer validate --strict
+
+      - name: Cache Composer packages
+        id: composer-cache
+        uses: actions/cache@v2
+        with:
+          path: vendor
+          key: ${{ runner.os }}-php-${{ hashFiles('**/composer.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-php-
+
+      - name: Install dependencies
+        run: composer install --prefer-dist --no-progress --ignore-platform-reqs
+
+      - name: Validate workflow files (YAML)
+        run: npm run validate:yaml
+
+      - name: Lint Plugin code (PHP)
+        run: composer run lint:php
+
+      - name: Lint Plugin scripts (JS)
+        run: composer run format:js:fix && composer run lint:js
+
+      #- name: Lint stylesheets (CSS)
+      #  run: composer run lint:css
+        
+      #- name: Lint templates (HTML)
+      #  run: composer run lint:html
+
+      - name: Lint documentation (Markdown)
+        run: composer run lint:md