Add theme option addModeClass (for language-aware themes)

Adds the theme option `addModeClass`, which enables the CM option `addModeClass` if set.
When enabled, themes can apply styles language-aware with rules like `.cm-m-javascript.cm-variable`.
CMs `addModeClass` uses innerMode for class name determination, so modes like htmlmixed, php or gfm work correctly, too.

See also #8579.
cc @MiguelCastillo 
I don't think so. The setting itself is not immediately user-visible (while custom scrollbars are) and some themes just wouldn't work without this setting.
So for example, IIRC one asked for this particular feature when he wanted to create an authentic-looking Notepad++ theme. Such a theme just wouldn't be possible without this option set.