feat: add custom-elements.json

[nolanlawson]

Fixes #31

I think this is mostly aspirational, as it doesn't seem like this file format is supported out-of-the-box by VS Code or WebStorm, but this post convinced me to go ahead and add it. Maybe it will do something useful in the future!

If nothing else, if IDEs ever add support for this and I did something wrong, at least there is a file now that people can edit and open a PR to fix.

[nolanlawson]

I can't seem to get this to work in VS Code or in WebStorm, and the <api-viewer> element doesn't support the 1.0.0 format yet (open-wc/api-viewer-element#61), and even when I run a basic element through @custom-elements-manifest/analyzer, the output doesn't perfectly agree with the TypeScript definition from @webcomponents / custom-elements-manifest (it says tagName does not exist on ClassDeclaration).

So it really feels like early days for this, but it's a great idea, so I'll add it anyway.

[nolanlawson]

Reported the bug about tagName. Keeping it for now because it might be an issue in the spec, not the analyzer? open-wc/custom-elements-manifest#43