Fix default export returning undefined in ESM

[Codex-]

I noticed this previously but it's consistently reproducible with Bun.

The esm based index.mjs export for concurrently, export default concurrently.default, in some cases depending on the way you're executing concurrently can have the default function on concurrently itself which makes this export not work as expect, as default is undefined.

This PR adds a simple fallback export default concurrently.default || concurrently that allows some defense against this behaviour.

I have added tests for both Node and Bun that test the commonjs and esm import behaviour, as well as a direct ts import case testing both for Bun's case.

Also, I've noticed Node 19 is targetted here but Node 20 enters LTS next month, should this target 20 instead of 19?

[coveralls]

coverage: 99.31%. remained the same when pulling ee85301 on Codex-:fix_bun_compatibility into 1618208 on open-cli-tools:main.

[gustavohenke]

Thanks for fixing this! I have mostly nitpicks, no questions on the intended change

[gustavohenke]

Is there a difference?

[Codex-]

non-zero exit codes will fail the CICD, there were cases in my own testing where simply using process.exit() would cause a false success on the workflow run

[gustavohenke]

Node logs the stacktrace by default, doesn't bun do the same? E.g. if you make the assertion fail, won't it print the stack trace twice?

[gustavohenke]

Why is catching the error necessary?

[Codex-]

unhandled rejections can fail silently, but these don't actually need to be async so I'll update them (ported from a larger suite I use for one of my own packages)

[paescuj]

Not sure if this is the preferred solution. I think this could be properly tackled by #399 instead.
Also I think it's about time to go full ESM, things are getting messy.
I'll look into this more detailed later on this day.