Some notes:

- This approach is not perfect, but it's rather simple and I can't
  think of an edge case.

- I did not use the `words` function to create the regex matching the
  keywords list, because it returns a capturing group (`()`) and it
  needs to be non-capturing here (because of `bygroups` usage).

- I chose to go to the 'soft-keywords-inner' state after both
  `match` and `case`, even though it's unnecessary for `match`
  (the inner state catches the `_` wildcard keyword which appears
  only after a `case`).

  This is mostly harmless and saves us from writing the 'soft-keywords'
  regex twice each for `match` and `case` with the extra inner state
  just for `case`.

  The only piece of code this will lex incorrectly is `match _:`
  (`_` will be lexed as keyword). I doubt though that pattern mathcing
  will be used like this.