Add multiview feature flag in compositing and constellation

[delan]

This patch adds a Cargo feature for the new multiple webviews work that is disabled by default.

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• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes are part of Multiple concurrent top-level browsing contexts #30648

• There are tests for these changes OR
• These changes do not require tests because there are no functional changes

[mrobinson]

Is there a reason that multiview should be behind a flag? Rendering to multiple web views is a pretty basic feature of a web engine. Is the issue that this work isn't complete yet?

[delan]

Yeah, in #30648 there have been some existing events with behaviour that we want to change, but we either need to change all in one go to avoid breakage, or the changes have bugs we still need to iron out.