[Glyphs] Support language statements containing multiple language tags

Same as https://github.com/googlefonts/glyphsLib/issues/1109

The urgency of this went up because apparently Glyphs 4 (stable released end of July) generates automatic features such as `locl` with this custom FEA extension syntax whereby `language` statement can multiple language tags (official FEA only allows one tag) and all lookups or inline rules following get registered under each of them.

https://github.com/googlefonts/glyphsLib/pull/1162#issuecomment-5539450250

Confirmed by the [changelog](https://updates.glyphsapp.com/Glyphs4.0-4000.html) as well

> Streamline generated fea code
> * put all languages into one language statement

Since the horse is out of the barn, fea-rs should just implement this and move on.

Start with the related glyphsLib issue and the linked Glyphs 4 changelog, then trace fea-rs's parser entry point for language statements. Check how following lookups and inline rules are registered, and identify the existing tests for language statements. Done means a statement with multiple language tags is accepted and subsequent rules are registered under each tag.

Written by the indexing model from the issue text.