Add ability to emit literal Typescript from with Go file

[pdhammond42]

This implements a feature to allow a Go file to contain Typescript definitions that can't be expressed through tstype tags.

There are actually two ways of doing it implemented. I think both have their uses but perhaps only one should be included, in which case I would say the var string literal one should be preferred.

Motivating Example: We have a struct that is marshalled into JSON, and therefore exposed to TS, as a tuple of values, to minimise size on the wire. (it saves about 30% after gzip compression)

Prior art: There are of course already ways to handle this.

• we could use frontmatter in the configuration, but I think this separates the TS declaration from the Go too much.
• we could simply hand-craft the relevant TS and refer to it in an import in frontmatter, but that's an even bigger separation.
• We could put comments into the Go that describe the mapping, which keeps it close but relies on the client reading and then using them.

I hope you find this change acceptable and thanks for reading.

[gzuidhof]

Let me know when you are happy for me to merge this

[pdhammond42]

Let me know when you are happy for me to merge this

I'm happy if you are happy with not handling const.