Generate widget style documentation

[tronical]

We should extend our parser to become aware of perhaps triple-slash comments before components, properties, and callbacks. Then the compiler could have a new output mode that would be "markdown" and that would be the content of these comments, along with tables/lists for the properties, and references to the docs of known types.

Initially, we could use this to generate the widget style documentation - by moving it from hand-written markdown to say the fluent style, generate the markdown and check in the generate markdown like we do for enums.

[hunger]

We should add //< at the same time to relate comments to the previous token. That is really handy in some cases.

I'll adapt my comment removal logic to only include these comments once this lands.

[tronical]

Duplicate of #104