[FR]: Support setting a default file extension

[Aghassi]

What is the current behavior?

Currently rules_swc defaults to js unless you have file types mts, mjs, cts, cjs. This means that you can't customize the file extension type out easily without doing a bunch of list comprehension. There are cases where you may not have individual files, so having the ability to just ask the rule to produce a different extension would be nice.

Describe the feature

Support passing something like

        default_ext = ".cjs",

to the swc_compile rule such that it renames all the files created on your behalf without the need for js_outs

We wrote a patch for this internally and would happily contrib it upstream if the maintainers are open to it.