Enable JSX mode: only update JSX syntax to use hyperscript. Does not transpile `import/export` statements. Works together with `jsx` to also transpile modules while transpiling JSX. When transpiling JSX, automatically insert at the top `import { h } from "preact"`. Same as `preact`, but imports from `＠externs/preact` `import { h } from "＠externs/preact"`.