Implement Indo-Europeanist characters

[topological-modular-forms]

As noted by Janus Bahs Jacquet:

much of what I typeset requires the font to support:

• characters used to transcribe Indo-Europeanist things (and various other commonly found transliterations), such as velars with inverted breves ⟨k̑ g̑⟩, vowels with inverted underbreves ⟨i̯ u̯⟩, vowels with macron + breve ⟨ā̆ ē̆ ō̆ ū̆ ȳ̆…⟩ macron + accent ⟨ā́ ḗ ī́ ṓ ū́…⟩ ** [Notice how, at least on macOS, the fallback font in the browser paints the macron and acute over the e differently to the others since ḗ is the only one that exists as a precomposed character]