Better detection of when window.crypto.subtle works (detection of secure context)

[jonasfj]

The webcrypto API is only available in "secure contexts". We should detect when this isn't the case, make sure we throw UnsupportedError with a description error message, and ideally a link to some documentation about "secure contexts".

• Webcrypto API docs on mdn

  • This says:

    Secure context: This feature is available only in secure contexts (HTTPS), in some or all supporting browsers.

• Secure Contexts on mdn

We could potentially write up a note about this in the "compatibility notes" section in the README.md.

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Also it'd be nice if we had some test cases for this. Those might have to be manual test cases, because dart test running in the browser will probably fake a secure context.