[v0.37] Full deprecation of .babelrc.js

[thedavidprice]

Continues work from #3231

Current implementation that shipped with v0.36:

• only applicable to API, i.e. api/.babelrc.js
• deprecates but does not throw or warn if file exists in an app

From Peter P:

I want this to be a breaking change if you have custom babel transformations. The reason I want it to be a breaking change is because I want to discourage people from adding their own babel transformations because I would like to move our own transformations to a faster toolchain and would not like babel to make that process difficult.

Next Steps

• Full deprecation: required to delete api/.babelrc.js and/or to convert it to api/babel.config.js and remove the "extends" part.
• Add tests:
  • Test that /.babelrc.js is not imported
  • Test that /babel.config.json can be imported if it's available.
• Resolve conversation here about rw alias plugin and supporting custom aliases: https://github.com/redwoodjs/redwood/pull/3231/files#r687869806
• TBD: also include Web web/.babelrc.js?

[peterp]

We use babel in 3 different scenarios:

1. Pre-build: we transpile the magic out of the code, and turn it into ordinary JS code.
2. Type generation/ Babel plugins: we "read" the user's source code to determine what's inside some file: Is it a Cell? Is it a directory-named-module? Does it have exports?
3. Real-time run-time transpilation of files: We convert your TypeScript code into beautiful runnable JavaScript code in memory (in the same node process). We use this for pre-rendering, "console", and data-migrate.

Each one of these functions need to use ✨THE SAME CONFIGURATION✨ because if we're doing one thing in pre-build and another in real-time... People are going to loose their minds. If we don't loose them first... I'm taking mutiny. I'm talking "let them eat cake."

So... I propose that we come up with unit tests for the functions above that confirm we're not modifying their configuration in ways that make them inconsistent with each other.

[thedavidprice]

@dac09 nudge on this one, high priority for v0.39