[babel 8] Better file ext handling for TS and Flow presets

[nicolo-ribaudo]

Q	A
Fixed Issues?	Fixes #1, Fixes #2
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link	babel/website#2773
Any Dependency Changes?
License	MIT

This PR moves #11316 from babel-8-dev to main. The first commit is #14954, please ignore it when reviewing this PR.

It provides a better configuration experience when using Flow, TypeScript or JSX. The two main changes are that the plugins/presets don't opaquely disable other plugins, and that a filename isn't required anymore. I also changed the options they take regarding file names:

• When using the TS preset, the filename is not required anymore
• When both the TS and Flow plugins are enabled on the same file, an error is thrown rather then giving the precedence to TS
• The TypeScript plugin doesn't disable JSX anymore
• By default, the Flow preset only enables the plugin when applied to a non-ts file
• The .isTSX option has been removed from the TS pluginsand preset, since users can directly enable the JSX plugin.
• The .allExtensions option has been removed from the TS plugin, since it was used to replace the extension test with the removed .isTSX option
• A new option, .ignoreExtensions, has been added to both the TS and Flow plugins, to make it possible to run them on JS and TS files respectively

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/54177/

[nicolo-ribaudo]

Merging with a single approval since this is just a backport of an already approved and merged PR.