rc ini: More leniently highlight section names
rc ini: More leniently highlight section names

Highlight every character between brackets, including more
brackets. This allows alternative constructs in INI files such as:

```
[section]
    [[subsection]]
```

This commit also only applies the appropriate face on the section
name itself, not the entire line (including hypothetical surrounding
whitespace characters).

danr/kakoune,casimir/kakoune,mawww/kakoune,lenormf/kakoune,lenormf/kakoune,casimir/kakoune,alexherbo2/kakoune,alexherbo2/kakoune,mawww/kakoune,alexherbo2/kakoune,jkonecny12/kakoune,Somasis/kakoune,Somasis/kakoune,danr/kakoune,mawww/kakoune,casimir/kakoune,danr/kakoune,jkonecny12/kakoune,lenormf/kakoune,danr/kakoune,jkonecny12/kakoune,alexherbo2/kakoune,Somasis/kakoune,jkonecny12/kakoune,Somasis/kakoune,mawww/kakoune,casimir/kakoune,lenormf/kakoune