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>

ports/servoshell/Cargo.toml

@@ -54,6 +54,7 @@ webgpu = ["libservo/webgpu"]
 webxr = ["libservo/webxr"]
 vello = ["libservo/vello"]
 vello_cpu = ["libservo/vello_cpu"]
+raqote = ["libservo/raqote"]
 
 [dependencies]
 cfg-if = { workspace = true }