Vendor rfc6902 package (#173)

[MarcusLongmuir]

Resolves #173.

The latest (5.1.1) release of the rfc6902 npm package does not have significant optimisations that were previously part of a fork of a fork that was a git dependency: https://github.com/MarcusLongmuir/rfc6902/tree/browser-compatible.

To address this regression the fork has been vendored into this codebase with an accompanying license.

This should significantly speed up EditableNetworkedDOMDocument reloading.

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What kind of changes does your PR introduce? (check at least one)

• [x] Bugfix

Does your PR introduce a breaking change? (check one)

• [x] No

Does your PR fulfill the following requirements?

• [x] All tests are passing
• [x] The title references the corresponding issue # (if relevant)