resourceloader: Soft-deprecate use of global LESS variables

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