resourceloader: Soft-deprecate use of global LESS variables

Add deprecation notice to $wgResourceLoaderLESSVars and hook 'ResourceLoaderGetLessVars'. Warnings to be added in 1.31. Users should migrate to exposing variables via the individual module's getLessVars() method. - Ensures better cache invalidation. - Separates concerns for getting information and exposing information as less variables. And allows an extension to, for example, use a different variable name for something if needed. The 'deviceWidthTablet' variable will need to be migrated to either a regular configuration variable that individual modules can expose in their getLessVars() method - or, alternatively, we can deprecate it altogether in favour of exposing it through a less import file instead (see T112747). That would mean, however, that the value can only be changed in core, not by site config. Bug: T140804 Change-Id: I61cff1d9652d88dc53c43075dd5053b7707809e6

Author: Krinkle

docs/hooks.txt

@@ -2764,9 +2764,10 @@ configuration variables to JavaScript. Things that depend on the current page
 or request state must be added through MakeGlobalVariablesScript instead.
 &$vars: array( variable name => value )
 
-'ResourceLoaderGetLessVars': Called in ResourceLoader::getLessVars after
-variables from $wgResourceLoaderLESSVars are added. Can be used to add
-context-based variables.
+'ResourceLoaderGetLessVars': DEPRECATED! Called in ResourceLoader::getLessVars
+to add global LESS variables. Loaded after $wgResourceLoaderLESSVars is added.
+Global LESS variables are deprecated. Use ResourceLoaderModule::getLessVars()
+instead to expose variables only in modules that need them.
 &$lessVars: array of variables already added
 
 'ResourceLoaderJqueryMsgModuleMagicWords': Called in

includes/DefaultSettings.php

@@ -3754,20 +3754,18 @@
  * at the beginning of all your .less files, with all the consequences.
  * In particular, string values must be escaped and quoted.
  *
- * Changes to LESS variables do not trigger cache invalidation.
- *
- * If the LESS variables need to be dynamic, you can use the
- * ResourceLoaderGetLessVars hook (since 1.25).
+ * Changes to this configuration do NOT trigger cache invalidation.
  *
  * @par Example:
  * @code
  * $wgResourceLoaderLESSVars = [
- * 'baseFontSize' => '1em',
- * 'smallFontSize' => '0.75em',
- * 'WikimediaBlue' => '#006699',
+ * 'exampleFontSize' => '1em',
+ * 'exampleBlue' => '#eee',
  * ];
  * @endcode
  * @since 1.22
+ * @deprecated since 1.30 Use ResourceLoaderModule::getLessVars() instead to
+ * add variables to individual modules that need them.
  */
 $wgResourceLoaderLESSVars = [
 /**