Added support for babel.config.js

[ross-weir]

Summary

Fixes #6908

With the arrival of Babel 7.0.0 comes support for .js configuration files from Babel.

See here for details.

We are introducing babel.config.js. It isn't a requirement or even a replacement for .babelrc, but having this may be useful in certain cases.

*.js configuration files are fairly common in the JavaScript ecosystem. ESLint and Webpack both allow for .eslintrc.js and webpack.config.js configuration files, respectively.

This PR adds support for the new config file added by the Babel team.

[thymikee]

Mind adding a changelog entry? :)

[ross-weir]

Added changelog entry. Thanks! :)

[codecov-io]

Codecov Report

Merging #6911 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6911      +/-   ##
==========================================
- Coverage   66.98%   66.98%   -0.01%     
==========================================
  Files         250      250              
  Lines       10360    10365       +5     
  Branches        3        3              
==========================================
+ Hits         6940     6943       +3     
- Misses       3419     3421       +2     
  Partials        1        1

Impacted Files	Coverage Δ
packages/babel-jest/src/index.js	88.7% <ø> (-2.52%)	⬇️

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[SimenB]

Note that this just adds support in our cache busting - we already supported the config file for actual configuration of babel (as we just ask babel to do the transpilation for us)

[miguel-orange]

@SimenB - Thanks for the comment, all I needed to do when migrating from .babelrc to babel.config.js is --clearCache, and jest picked up the js.

[capsule5]

Is it possible to access babel.config.js from a custom path?

[SimenB]

Yeah, using a custom transformer: https://jestjs.io/docs/en/tutorial-react#custom-transformers

[github-actions]

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