fix(web): make start and resume async

[Gustl22]

Description

If playing an invalid file (e.g. an empty file), the error is not propagated back to the library, as the error is thrown when starting the play / resume operation.
This ensures that the future of web js play is awaited.

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs:, chore: etc).
• I have read the Contributor Guide and followed the process outlined for submitting PRs.
• I have updated/added tests for ALL new/updated/fixed functionality. Tests follow in feat!: event channel #1352
• I have updated/added relevant documentation and added dartdoc comments with ///, where necessary.
• I have updated/added relevant examples in example.

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.

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