Use babel to transpile all non-vendor javascript assets regardless of file extension

What does this MR do?

Turns on babel processing for all javascript file extensions, not just *.js.es6. I've tested the output and the production compiled application.js differs by only 12 bytes.

Are there points in the code the reviewer needs to double check?

Make sure no tests break (they shouldn't)

Why was this MR needed?

It's silly that we need to use different file extensions, and eslint doesn't understand this naming scheme anyway.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#27486 (moved)