Do not change the original file extension

**DISCLAIMER:** I know that this issue has already been [proposed and rejected different times](https://github.com/babel/gulp-babel/issues?q=is%3Aissue+sort%3Aupdated-desc+is%3Aclosed+extension).

`gulp-babel` always renames the file to `.js`, regardless of the input extension. This was the most reasonable behavior until a few years ago: `.js` was the only extension for JavaScript.

However, now there are three "official" (supported by Node.js) JavaScript extensions:
- `.js`
- `.cjs`
- `.mjs`

and renaming them to `.js` is undesirable, since they are not interchangeable. I think that this is a good reason to reopen the discussion, and we have two options:

1. Add a `keepFileExtensions` option, that matches [`@babel/cli`'s `--keep-file-extensions`](https://babeljs.io/docs/en/babel-cli#set-file-extensions)
2. (breaking) Do not support changing the extension, and let users use [`gulp-rename`](https://www.npmjs.com/package/gulp-rename)

I am aware that there is the [`gulp-revert-path`](https://www.npmjs.com/package/gulp-revert-path) plugin that undoes Babel's extension change, but it is a workaround and not a proper solution: while a workaround made sense when `.js` was the only reasonable extension, I think we need a real solution now.

*Context: https://github.com/babel/babel/pull/12820*

Review the prior rejected proposals and the Babel PR #12820 context before choosing a direction. Compare the proposed keepFileExtensions option with the breaking behavior change, and consider how each relates to gulp-rename and gulp-revert-path; done means an agreed solution for preserving .js, .cjs, and .mjs semantics.

Written by the indexing model from the issue text.