* The previous convention of !* ... *! are now true block comments,
  i.e. they are parsed faster, don't spam the goto table and allow
  embedding of exclamation marks - only "*!" terminates the comment.
* It is therefore now forbidden to have goto labels beginning with "*".
* Also support "!!" to introduce EOL comments (like C++'s //).
  This disallows empty labels, but they weren't useful anyway.
  This is the shortest way to begin a comment.
* All comment labels have been converted to true comments, to ensure
  that syntax highlighting works correctly.
  EOL comments are used for single line commented-out code, since it's
  easiest to uncomment - you don't have to jump to the line end.
  This is a pure convention / coding style.
  Other people might do it differently.
* It's of course still possible to abuse goto labels as comments
  as TECO did for ages.
* In lexing / syntax highlighting, labels and comments are highlighted differently.
* When syntax highlighting, a single "!" will first be highlighted as a label
  since it's not yet unambiguous. Once you type the second character (* or !),
  the first character is retroactively styled as a comment as well.

* This is optimized for Groff, but works for Heirloom Troff and Neatroff as well.
  Currently, the Heirloom and Neatroff requests are just added ontop of the Groff
  ones. Theoretically, we could also try to separate the keyword lists into
  a base K&R set with Groff, Heirloom and Neatroff ontop.
* The lexer necessarily has many restrictions, as Troff is fundamentally unparseable
  (like classic TECO) and needs a lot of per-request knowledge.
* The "*.mm" extension has been removed from the lexers/cpp.tes.
  I don't know what language this was for, and I prefer `*.mm` files
  to be considered Troff.
* Temporarily changed the lexilla submodule URL.
  The corresponding Lexila lexer is in the process of being upstreamed.
  Once it is, I will probably revert the submodule to the official repository,
  as the "troff" branch is not stable (can be rebased).