tweak(ocaml): add more extra ligatures

[Dev380]

Tuareg mode (for ocaml) current uses
its own ligatures. This extends that
with some extra, non-conflicting ones
using the :ui ligatures module's
pre-defined ligatures.

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• I searched the issue tracker and this hasn't been PRed before.
• My changes are not on the do-not-PR list for this project.
• My commits conform to the git conventions.
• My changes are visual; I've included before and after screenshots.

Before:

After:

I'm using the dot instead of the cartesian product sign for the tuple type because it uses the * character, which is also used for multiplication. One way to resolve this is to add the cross product symbol to the default +ligatures-extra-symbols plist as a compromise between X for multiplication and X for cartesian product. I'm not sure if this is optimal, though, should this be done?

I added the tuple symbol just for the screenshot, but I might remove it because of how hacky it is (matches on ", " to avoid it being lopsided, but then tuples not in that format won't work + there are no spaces on each side).