Support typing Unicode for all symbols

[jared-hughes]

E.g. you can type/paste "θ" and get "\theta" instead of the raw unicode θ. Inside Desmos, this means you can paste r=\sin\left(θ\right) and get a valid equation instead of "Sorry I don't understand the 'θ' symbol."

There was already some support for this, e.g. →, π, ≤, but it was missing for all Greek letters and many advanced symbols. My plan was to just do the Greek letters, but I got carried away.

For each added unicode support, I verified:

• (by grep) The unicode appears only once in the source (e.g. LatexCmds['∇'] only appears once)
  • This ensures that there's no advancedSymbols definition overwriting a basicSymbols definition.
• (by pasting into a mathquill field) The mathquill glyph for the corresponding command roughly matches my local glyph (from font Julia Mono) when the unicode is pasted into a mathquill field
  • this ensures I didn't accidentally swap a ⊆ and ⊇

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Future PR, if we care:

• \parallel, \nparallel, and \perp are in both advancedSymbols and basicSymbols
• advancedSymbols has \bigtriangledown but not \triangledown.