Add Clipboard interface, readText, writeText

[garyb]

Description of the change

See title 😄

---

Checklist:

• Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation
• Added a test for the contribution (if applicable)

[JordanMartinez]

Can you post the link to the spec?

[garyb]

Oh sure, sorry.

• Here's the modification to the Navigator interface to add clipboard: https://w3c.github.io/clipboard-apis/#navigator-interface
• Here's the Clipboard interface itself: https://w3c.github.io/clipboard-apis/#clipboard-interface

I perhaps also should have mentioned this is a partial implementation of the API, I need writeText but don't have time to figure out all the ClipboardItem stuff so just included the plain text operations. Taking the lazy approach where implementation is forced by need 😉

[JordanMartinez]

Can you add a changelog entry?