[webcomponents] Style edits to Type-Extensions.md (#118)

Reformat contributions made in #117 to better match the rest of the document.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/webcomponents/pull/118

-- Commit Summary --

  * Style edits to Type-Extensions.md

-- File Changes --

    M proposals/Type-Extensions.md (96)

-- Patch Links --

https://github.com/w3c/webcomponents/pull/118.patch
https://github.com/w3c/webcomponents/pull/118.diff

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/pull/118