xilem_web: Add Rotate and Scale (CSS) transform views

[Philipp-M]

This sketches/implements (parts of) the transform modifier API described in Add Affine transform to Widget trait? for xilem_web.

This currently includes rotate and scale, because there are less cases to handle for these CSS transform functions.
In rotate I think we can reduce this to just radians (there are more units for <angle>: turn, grad, deg and rad (used here) but they can all be derived from radians), and percent in scale is kinda redundant (as 200% == 2.0)), for example translate has more cases as it includes all kinds of units (like px, em, % etc.), so this may require more thought (and is thus probably better for a future PR).

This can be combined with untyped transform: ... as style, these modifier views extend the transform style (while the untyped style(..) overwrites previous set values).

The svgtoy example is updated to include these views.

[flosse]

LGTM!

Just some random thoughts:
Maybe in the future we can add types that handle different units like degrees, etc, but that's probably not where we're at here (users can use an external crate for proper unit handling). Or we could add a Radians type so that the user at least knows what is being used here, although this might be an unnecessary thing that just makes things cumbersome.

[Philipp-M]

Or we could add a Radians type so that the user at least knows what is being used here

I think default radians and an additional degrees (to radians) function may be better, radians are usually playing better with other related math (e.g. trigonometric functions).

Anyway, I've removed the macro style_impls, it's not 100% correct TBH, as MathML styles are not (yet) working (should work after a follow-up PR, when rustwasm/wasm-bindgen#4143 is merged though). But I think I would just accept that for now, as I don't think macros are a good choice there (jump to definition with rust-analyzer e.g.). The possibility that this API is used with MathML elements is negligible I think.
I've also added style size hints, so that we avoid unnecessary allocations when creating these modifiers...

Thanks for the review, AFAICS there weren't objections in office hours, so I guess we can start experimenting with this kind of modifier API in xilem_web (with a potential future in xilem itself), so I'll merge this.