[webcomponents] Add Type-Extensions.md (#114)

Bare skeleton to be fleshed out.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/webcomponents/pull/114

-- Commit Summary --

  * Add Type-Extensions.md

-- File Changes --

    A proposals/Type-Extensions.md (34)

-- Patch Links --

https://github.com/w3c/webcomponents/pull/114.patch
https://github.com/w3c/webcomponents/pull/114.diff

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/pull/114