Add hb_font_funcs setter functions

[madig]

Make hb_font_funcs_set_* accessible from the JavaScript side, with HB being able to call JavaScript functions the user passed in.

Had a quick look at this, but failed to understand the emscripten docs and various StackOverflow answers...

[justvanrossum]

We will need this, too, at some point. Specifically these:

• hb_font_funcs_set_nominal_glyph_func
• hb_font_funcs_set_glyph_h_advance_func
• hb_font_funcs_set_glyph_v_advance_func
• hb_font_funcs_set_glyph_v_origin_func

I am brand new to this project (although I have used and contributed to the equivalent parts of uharfbuzz), and hope to find my way so I can contribute.

In the meantime, some guidance about how to approach this would be very welcome.

[khaledhosny]

You can see an example of setting drawfuncs here: https://github.com/aliftype/rana-kufi/blob/99f9ba560e3c4a6d858d2b9d2c807466fb975c49/docs/app/HarfBuzz.js#L116

[khaledhosny]

See #97