Define sharable scheme + check

[marcoscaceres]

Closes #178

For normative changes, the following tasks have been completed:

• [ ] Modified Web platform tests (link to pull request)

Implementation commitment:

• [x] WebKit (https://bugs.webkit.org/show_bug.cgi?id=)
• [ ] Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=)
• [ ] Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=)

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[marcoscaceres]

thanks for checking/approving it @ericwilligers. I'm adding a few WPTests.

[marcoscaceres]

Actually, turned out we have tests already. https://github.com/web-platform-tests/wpt/blob/master/web-share/share-url-invalid.https.html