CLI: toggle JS/TS output

We want to add some logic to toggle TS output on/off in the CLI commands, and select .ts/.js (or .jsx/.tsx) based on that. To do that:

• have the core generators always emit TS, and add a MitosisConfig["outputs"]: ["typescript" | "javascript"] flag to the CLI.
  • If the outputs array contains typescript, we output the TS code as-is.
  • If the outputs array contains javascript, an additional transpilation step is executed that takes the TS code and transpiles it down to JS, and saves that to a file

The outputs array can contain 1 or both of those flags. If it’s empty, we default to typescript 🤔.

PS: We can probably reuse this function for transpilation: https://github.com/BuilderIO/mitosis/blob/ab0708fd2c07a06a7a52ab7490cd8e716759a48a/packages/cli/src/build/helpers/transpile.ts#L17

_Originally posted by @samijaber in https://github.com/BuilderIO/mitosis/issues/473#issuecomment-1154248996_

[kylecordes]

Yes, exactly. I definitely don’t urge growing scope too fast… but it would be perfect if eventually the config/flags included telling Mitosis what version of React/etc to use, then Mitosis would emit the package.json and all other needed files, ready to use with no further edits!

[samijaber]

It could be simpler than I am anticipating! We will soon switch the Builder SDKs to the ts output, and then build js+.d.ts files from that so that the final result has types. If I see that the work needed there is easily portable, we can definitely bake it into the CLI.