BREAKING: don't parse JSX in .js and .ts files

[ematipico]

No description provided.

[arendjr]

For .ts I understand, since the TypeScript compiler makes the same distinction, but why would we do this for .js files? There are plenty of codebases that use .js files with JSX content and it's an accepted practice.

[ematipico]

It's an old and debatable practice. Nowadays, new projects always use .tsx and .jsx straightaway. New and modern projects should rely on file extensions and prior config, see .mjs and .cjs and pakcage.json#type, for example.

We intend to provide an option to opt in to the old behaviour (there's already a task for that in the checklist).