Improve helper system

Add getHelpers and registerGlobalHelper, allow modes to specify arrays
of helper names, support modeProps in mode specifications.

Update foldcode and show-hint to make use of more powerful helper
system by defining an `auto` helper that tries multiple helpers, and
using that as default.
[css mode] Refactor, step 1 (tokenizer)
[css mode] Refactor, step 2 (state machine)
[css mode] Refactor, step 3 (Less support)
[css mode] Add a hintWords helper value
[css-hint addon] Rewrite to work with overhauled css mode