core-commit-message.txt

Editor: Add CodeMirror-powered code editor with syntax highlighting, linting, and auto-completion.

* Code editor is integrated into the Theme/Plugin Editor, Additional CSS in Customizer, and Custom HTML widget.
* The CodeMirror component in the Custom HTML widget is integrated in a similar way to TinyMCE being integrated into the Text widget, adopting the same approach for integrating dynamic JavaScript-initialized fields.
* Linting is performed for JS, CSS, HTML, and JSON via JSHint, CSSLint, HTMLHint, and JSONLint respectively. Linting is not yet supported for PHP.
* When user lacks `unfiltered_html` the capability, the Custom HTML widget will report any Kses-invalid elements and attributes as errors via a custom Kses rule for HTMLHint.
* When linting errors are detected, the user will be prevented from saving the code until the errors are fixed, reducing instances of broken websites.
* Code editor is not yet integrated into the post editor.
* Placeholder value is removed from Custom CSS in favor of a fleshed-out section description which now auto-expands when the CSS field is empty. See #39892.
* Introduces `wp.codeEditor.initialize()` in JS to convert a `textarea` into CodeMirror, with a `wp_enqueue_code_editor()` function in PHP to manage enqueueing the assets and settings needed to edit a given type of code.
* User preference is added to manage whether or not "syntax highlighting" is enabled. The feature is opt-out, being enabled by default.
* Update allowed file extensions in theme and plugin editors to include formats which CodeMirror has modes for: `conf`, `css`, `diff`, `patch`, `html`, `htm`, `http`, `js`, `json`, `jsx`, `less`, `md`, `php`, `phtml`, `php3`, `php4`, `php5`, `php7`, `phps`, `scss`, `sass`, `sh`, `bash`, `sql`, `svg`, `xml`, `yml`, `yaml`, `txt`.

Props westonruter, georgestephanis, obenland, melchoyce, pixolin, mizejewski, michelleweber.
See #38707.
Fixes #12423, #39892.