Support ECMAScript modules (ESM)

[davidtheclark]

This PR intends to close https://github.com/davidtheclark/cosmiconfig/issues/224. I don't have more time to work on this today so I'll get what I have up as a draft — feedback welcome! I'd especially appreciate help testing this from people interested in using ESM, both ensuring that it works as expected and ensuring that the expectations are the right ones.

Some notes:

• ESM is only supported by the asynchronous API, because it is enabled by the asynchronous import() function.
• So .mjs is only added to the default searchPlaces of the asynchronous API.
• I turned off the selective branches in CI configuration because I wasn't seeing tests run. It may be that Travis and Appveyor needed to catch up with the main branch name change (from master to main). We'll see.
• I was having some trouble early on with Jest, so tried using the jest-circus test runner. Turns out that wasn't the problem at all, but I left in that update.
• This PR is set to merge into a v8 branch, in case there's anything else in the issue queue that anybody wants to get into v8.

[codecov-io]

Codecov Report

Merging #251 (7c28c3e) into v8 (1834605) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##                v8      #251   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines          227       235    +8     
  Branches        46        47    +1     
=========================================
+ Hits           227       235    +8     

Impacted Files	Coverage Δ
src/ExplorerBase.ts	100.00% <ø> (ø)
src/index.ts	100.00% <100.00%> (ø)
src/loaders.ts	100.00% <100.00%> (ø)

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[davidtheclark]

Known to-dos:

• [ ] Document change to defaultLoaders (loadJs is async) and exposure of defaultLoadersSync. Or consider changing this. And maybe we want to address https://github.com/davidtheclark/cosmiconfig/issues/226.
• [ ] Write new tests that find ESM configs.

[davidtheclark]

By using the --experimental-vm-modules flag and some workarounds for the inability to clear the ESM loader cache, the tests are now passing for me locally. But on Travis, they are not, with segmentation faults in Node 10 and 14 and something else in Node 12 I don't understand yet.

[perrin4869]

The segfault issue is probably caused by https://github.com/nodejs/node/issues/35889

[davidtheclark]

Just posted this note at the top of the README:

MAINTAINERS WANTED! If you're interested in taking over and maintaining Cosmiconfig, please let @davidtheclark know (with an issue or email). I'd like to hand over the keys completely, so I'm looking for owners, not people who just want to merge a PR or two! You can make the decisions about what happens in v8 and subsequent versions, how the package balances stability and opinionated features, and so on. Take a look at open issues and PRs to learn about possibilities that have been on people's minds over the years.

If you're interested in taking ownership of the package and moving this feature forward, let me know!

[d-fischer]

This was superseded by #304, which is now merged.