Attempt to add Math.atan2 to the inbuilt math function

[NigelBreslaw]

[x] Add builtin. [x] Update docs. [ ] Add test.

[ogoffart]

Don't forget to make use of it in a test, such as tests/cases/expr/atan.slint or tests/cases/expr/trigo.slint

[FloVanGH]

worth for a changelog entry ;-) @NigelBreslaw

[NigelBreslaw]

worth for a changelog entry ;-) @NigelBreslaw

Done!