canvas: Gate raqote backend behind feature (enabled for now) (#38312)

We want to eventually remove raqote backend, but for now we can gate it
behind a feature (still enabled by default in servoshell) like the rest
of backends. `dom_canvas_backend=auto` will select first available
backend. Builds on top of https://github.com/servo/servo/pull/38310 to
support cases where no backend is available.

Testing: It compiles with or without feature

---------

Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>

components/constellation/Cargo.toml

@@ -18,6 +18,7 @@ tracing = ["dep:tracing"]
 webgpu = ["script_traits/webgpu"]
 vello = ["canvas/vello"]
 vello_cpu = ["canvas/vello_cpu"]
+raqote = ["canvas/raqote"]
 
 [dependencies]
 background_hang_monitor = { path = "../background_hang_monitor" }