support "jsx": "preserve" tsconfig?

[isaacs]

Right now, tshy assumes that .tsx will always be built to .js. However, if the tsconfig has "jsx": "preserve", then that won't be the case, it'll be .jsx instead.

[isaacs]

Going to do this as soon as someone comes along and says they need it (or submits a PR to do it). Until then, this issue can just hang out and avoid the complexity, because I use React for all my JSX anyway.

[DaniGuardiola]

I plan on using tshy to compile some Solid.js code, so I guess I'll need this.