Finish the integration of webxr into the Cargo workspace (#35229)

- Run `cargo fmt` on `webxr` and `webxr-api`
- Fix clippy warnings in the existing `webxr` code
- Integrate the new crates into the workspace
- Expose `webxr` via the libservo API rather than requiring embedders to
  depend on it explicitly.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

Cargo.lock

@@ -6884,7 +6884,6 @@ dependencies = [
  "tracing-subscriber",
  "url",
  "vergen-git2",
- "webxr",
  "windows-sys 0.59.0",
  "winit",
  "winres",
@@ -8605,6 +8604,7 @@ dependencies = [
  "ipc-channel",
  "log",
  "serde",
+ "time 0.3.37",
 ]
 
 [[package]]