Put the Gamepad API and its supporting infrastructure behind a `gamepad`
feature flag. This allows embedders to opt-out of gamepad support at
compile time to save on binary size and reduce dependencies.
Testing:
1. `./mach build -d` (Gamepad enabled by default)
2. `cargo build -p servoshell --no-default-features --features
"libservo/clipboard,js_jit,max_log_level,webgpu"` (Gamepad Disabled)
3. `cargo build -p servoshell --features "gamepad,webxr,..."` (Gamepad &
WebXR Enabled)
Fixes: #40897
Signed-off-by: WaterWhisperer <waterwhisperer24@qq.com>
