Add async isSupported to Modular FM SDK

[zwu52]

This PR does 2 things

• export async isSupported from index.ts and index.sw.ts (API Approved: go/js-messaging-is-supported)
• separate isWindowSupported and isSwSupported checks in its own context (window and sw respectively)

Deprecating #4446 in favor of this PR

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Latest commit: ed6fa88

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[google-oss-bot]

Size Analysis Report

Affected Products

Diffs between base commit (f24d896) and head commit (f16f6d0) are too large (196,985 characters) to display.

Please check below links to see details from the original test log.

Test Logs

• Base (f24d896): https://github.com/firebase/firebase-js-sdk/actions/runs/724144719
• Head (f16f6d0): https://github.com/firebase/firebase-js-sdk/actions/runs/727251591

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