[Feature request] raw mouse input, no acceleration, with `requestPointerLock({ unadjustedMovement: true })`

**Version of emscripten/emsdk:**
3.1.58

This affects shooter games especially. Chromium introduced the [`unadjustedMovement`](https://developer.mozilla.org/en-US/docs/Web/API/Element/requestPointerLock#unadjustedmovement) parameter not so long ago, which makes aiming so so much better, indistinguishable from running a native game.

There is an example of an app, Quake III Arena, that utilizes Emscripten with a hack to get it to use `unadjustedMovement`:

https://github.com/jdarpinian/ioq3/blob/7801abd5ac510a889087ad62473c54dbe509e106/code/web/index.html#L301-L322

It can be tested here: https://thelongestyard.link/

However, I believe that this should be upstreamed to Emscripten itself. Perhaps with a config option, if needed.

As of 2025-06 it's only supported on Windows and macOS, however, and its support needs to be detected, perhaps with `try { await canvas.requestPointerLock({ unadjustedMovement: true }) catch ...`.
Also note that this parameter is considered experimental.

I wanted to try to make an MR, but I am not sure where exactly this needs to be changed (there is [a lot of](https://github.com/search?q=repo%3Aemscripten-core%2Femscripten+requestPointerLock&type=code) `requestPointerLock` references), and how to handle the fact that this option might not be supported.