* Adds `WP_Customize_Code_Editor_Control` and `wp.customize.CodeEditorControl()`.
* Control respects user preference for syntax highlighting, showing a textarea when user opts out.
* Code editor control takes the ad hoc code for Additional CSS and makes it reusable and extensible, for Additional CSS in core and plugins to use (such as Jetpack).
* Replace `settings` arg in `wp_enqueue_code_editor()` with separate args for `codemirror`, `csslint`, `jshint`, and `htmlhint`.
* Prefix `codemirror` script and style handles with `wp-` to prevent collisions, as also the object is exported as `wp.CodeMirror` in JS.
* Reduce indent size in Customizer code editor instances and Custom HTML widget to use tab size of 2 instead of 4 to save on space.
See #12423, #38707, #35395.
Fixes#41897.
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* Code editor is integrated into the Theme/Plugin Editor, Additional CSS in Customizer, and Custom HTML widget. Code editor is not yet integrated into the post editor, and it may not be until accessibility concerns are addressed.
* 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.
* The 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.
* The CodeMirror library is included as `wp.CodeMirror` to prevent conflicts with any existing `CodeMirror` global.
* An `wp.codeEditor.initialize()` API in JS is provided 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.
* A user preference is added to manage whether or not "syntax highlighting" is enabled. The feature is opt-out, being enabled by default.
* Allowed file extensions in the theme and plugin editors have been updated 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, afercia, grahamarmfield, samikeijonen, rianrietveld, iseulde.
See #38707.
Fixes#12423, #39892.
Built from https://develop.svn.wordpress.org/trunk@41376
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Ensures that users who copy HTML from the Text widget in legacy mode over to the Custom HTML widget will continue to get all of the same filters applied, including tag balancing and shortcodes, if a plugin added support. Plugins still have the `widget_text_content` and `widget_custom_html_content` filters they can use to target the specific widget types.
Amends [40893].
See #40951.
Fixes#40907 for trunk.
Built from https://develop.svn.wordpress.org/trunk@41086
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