Get rid of hacks for detecting languages that use a CM mode configuration

[njx]

Currently, we have hacks in various places to deal with the fact that we can't easily map back from a CM mode configuration (e.g. CSS mode configured for LESS, or XML mode configured for HTML) to an actual language. We should work with Marijn to figure out a way for CM to expose to us what the actual specific MIME type is for a given location, as opposed to just giving us the main mode plus a configuration bag.

[njx]

To @peterflynn

[njx]

Examples of the hacks we have today:

• code at the end of LanguageManager to deal with XML/HTML mode and clike/PHP mode
• code in the LESS language registration to deal with CSS/LESS mode (added in Return LESS/SCSS instead of CSS language when in a LESS/SCSS file #7346)
• code in TokenUtils.getModeAt() to handle XML mode specially