Disable letter-spacing for Arabic text (issue #4208)

[ChrisLoer]

Disables letter-spacing for Arabic text as discussed in issue #4208.

Two things to note:

• The decision to space or not to space is done on a per-feature level, instead of being done at word boundaries, which might make more sense for diglossic labels. I could make an argument that consistent spacing within the feature is a good thing, but mainly this is just significantly easier to implement.
• The PR only addresses Arabic, where there's a known problem. I haven't done an inventory of other scripts where this might be a problem. To be fully aware of when letter spacing is allowed, we'd need to work with the font (i.e. using Harfbuzz to render text).

/cc @1ec5 @nickidlugash @jcsg

[1ec5]

The PR only addresses Arabic, where there's a known problem.

That’s fine, since Arabic is the only writing system for which we support ligatures at the moment.

[nickidlugash]

LGTM 👍