Improve API of `Vec2`, `Mat2`, `Vector` and `Matrix` in the next major version

[fwcd]

Specifically...

• Using the initializer style where possible (e.g. Mat2.init(NDArray) should be preferred over NDArray.asMat2)
• Making .zero and .identity a property/constant in Vec2 and Mat2, to be consistent with AdditiveArithmetic
• Renaming Vec2.mapBoth to .mapEach (since mapBoth could also be read as applying to both components at once)
• Possibly renaming T to Element to be consistent with Swift types like Array (see e.g. here)
• Conforming the types to Sendable if T: Sendable

Internally, the types could also be refactored, e.g.

• By implementing elementwise arithmetic in terms of map/zip (like e.g. Coordinates from swift-geodesy)