Handle VHDL styles in `highlighting_is_{comment,string}_style()`

Closes geany/geany-plugins#123 You can view, comment on, or merge this pull request online at:

https://github.com/geany/geany/pull/1402

-- Commit Summary --

* Properly categorize VHDL strings and comments

-- File Changes --

M src/highlighting.c (9)

-- Patch Links --

https://github.com/geany/geany/pull/1402.patch https://github.com/geany/geany/pull/1402.diff