Re: [webcomponents] [Custom]: "are" custom element names ASCII characters, or MUST they be ASCII characters? (#239)

*sigh* I'm not sure I want to work on this, there's five sets of names, as far as I can tell, of which three are used (with two of them arguably wrong):

1. HTML parser names. [a-z] for the first letter followed by pretty much anything.
2. [xml 4th edition Name](https://www.w3.org/TR/2006/REC-xml-20060816/#NT-Name). I think most browsers use this for `createElement()`.
3. [xml 5th edition Name](https://www.w3.org/TR/2008/REC-xml-20081126/#NT-Name). Technically what browsers should use for `createElement()`, but don't. This is what allows emojis.
4. [xml-names 2nd edition NCName](https://www.w3.org/TR/2006/REC-xml-names-20060816/#NT-NCName). Used for `createElementNS()` and elements in the XML parser.
5 [xml-names 3th edition NCName](https://www.w3.org/TR/2009/REC-xml-names-20091208/#NT-NCName). Should be used for `createElementNS()` and elements in the XML parser.

I see two sane approaches here:

1. We restrict custom elements to ASCII alpha + ASCII hyphen.
2. We follow the restrictions from `createElement()` and `createElementNS()`, while ignoring that those are different from each other, from the HTML parser, and from what should be implemented for them per the latest XML specifications. (This requires no restriction to be specified and defers this mess to be cleaned up by the next generation, likely still us.)

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