JSX and TSX support

[ryanatkn]

tsv's current stance is to not support JSX/TSX, as it's designed to support Svelte without compromises. I'm open to measuring the outcomes of PRs/forks adding JSX support, and I have some in-progress design work on how this could look, but it's low priority and I'm wary of the costs even if relatively small.

Two mitigations: separate builds could be a thing, and in general devs should have a good experience forking tsv and reusing its parts for different goals, so a fork adding JSX may be the best outcome.