Draft spec for custom clipboard format.

[snianu]

Closes #????

The following tasks have been completed:

• [X] Confirmed there are no ReSpec/BikeShed errors or warnings.

Implementation commitment:

• [ ] WebKit (https://bugs.webkit.org/show_bug.cgi?id=)
• [X] Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=106449)
• [ ] Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=)

[snianu]

Abandoning this PR since we decided to include the custom format in the official clipboard spec.