[webcomponents] Proposals directory contains non-Windows-compatible filenames (#111)

This makes it hard to clone the repo using Git on Windows.

```
proposals/Cross-Origin-Custom-Elements:-Concept-and-Proposal.md
proposals/Default-Stylesheets:-Concept-and-Proposal.md
"proposals/Limiting-the-superpowers-of-the-\"Shadow-DOM-piercing-cominator\".md"
"proposals/Multiple-Shadow-Roots-as-\"a-Shadow-Root-hosts-another-Shadow-Root\".md"
```

basically using : and " is bad.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/111