[glass] Add math expression input for NetworkTables numerical values

[rmheuer]

This is for easier adjustment of NetworkTables values when tuning values. For example, you can increase an angle by 5 degrees easily by adding +5 to the value instead of doing the addition yourself. We implemented a similar feature in our custom dashboard program, which we found very convenient during the build season.

[rmheuer]

/format

[PeterJohnson]

This is amazing work! It will take me a bit of time to fully review the expression parsing code.

[KangarooKoala]

Overall, looks pretty good! Just have a few relatively minor comments.

[rmheuer]

I think this is pretty much done now, do you have any other comments?

[KangarooKoala]

Might be worth it for Token to store std::string_view instead of const char* and int, but it doesn't matter much.

[KangarooKoala]

It would be really nice to have unit tests of the expression parser (e.g. does 1 + 2 ** 5 * 3 evaluate to 97), but glass doesn't have tests setup so it might not be worth it or be outside the scope of this PR.
Great work, though!