Fix type narrowing of image source types

[ezzatron]

This is a draft PR attempting to fix #13220. I don't think it's the best approach, but it might be useful for discussions.

The core problem of #13220 is that the new ImageSource type is serving double-duty as the basis for the CanvasSource and VideoSource types. This means that the type property is currently typed as string, which prevents type narrowing from working on the Source type.

The solution here does work, but having ImageSource be a generic type is probably not the nicest way to do it. Perhaps the ImageSource type should be separated from its implementation?

Launch Checklist

• Make sure the PR title is descriptive and preferably reflects the change from the user's perspective.
• Add additional detail and context in the PR description (with screenshots/videos if there are visual changes).
• Manually test the debug page. (N/A: no behavioural changes)
• Write tests for all new functionality and make sure the CI checks pass. (I wasn't able to because Vitest type tests don't seem to work in this project's test suite)
• Document any changes to public APIs. (N/A: no behavioural changes)
• Post benchmark scores if the change could affect performance. (N/A: no behavioural changes)
• Tag @mapbox/map-design-team @mapbox/static-apis if this PR includes style spec API or visual changes. (N/A)
• Tag @mapbox/gl-native if this PR includes shader changes or needs a native port. (N/A)

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[stepankuzmin]

Hi @ezzatron, thanks for opening PR for this! While I agree that making ImageSource generic is not the nicest way to achieve proper narrowing, I decided to do the same as you proposed in this PR.

I'm closing this PR since this will be available in the upcoming v3.5.2 #13230