Auto merge of #25694 - kunalmohan:24891-Constellation, r=Manishearth

Remove dependency of constellation on canvas

move `ConstellationCanvasMsg` to canvas_traits and start canvas paint thread
to components/servo. This, however, does not remove dependency for conditional
compilation options.

https://github.com/servo/servo/blob/5f55cd5d71df9c555fbc24777168396ddd539f28/components/constellation/Cargo.toml#L13-L15

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- [X] These changes fix #24891  (GitHub issue number if applicable)

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