enhanced string interpolations

[rmg]

Sorry for the giant code drop... unfortunately it's almost a complete re-write.

I started by just trying to fix multi-line comments but ended up adding a whole bunch more:

• booleans
• number literals, including supported suffixes
• syntax highlighting inside string interpolations
  • functions
  • variables
  • literals
• support for heredoc strings
  • embedded syntax hightlighting based on token suffix (<<CONFIG_JSON, <<CONFIG_YAML, ...)
  • string interpolation highlighting inside the embedded language (this one is kind of magical)

[samdunne]

Any chance we could look at merging this?

[GiantToast]

Merged and published.