Add isSupported for messaging-compat

[zwu52]

Sticky situation for providing legacy isSupported method with the modular SDK construct. Please let me know if there is a better way to just include and export the legacy API code in the messaging-compat. The rational for doing so is as followed:

• Modular isSupported became async. It's not compatible w/ the legacy sync version.
• Modular SDK has two entry points window and sw, but export isWindowSupported and isSwSupported as isSupported depends on the context. Not sure if there is a way to import isWindowSupported and isSwSupported separately from @firebase/messaging-exp to messaging-compat.

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[hsubox76]

I think this seems reasonable since we can't introduce breaking changes in compat.