Making migrationFileExtension: accept several extensions at the same time (.ts and .js)

[Alounv]

Is your feature request related to a problem? Please describe.

We have been using migrate-mongo from several years and it works fine. To this day our migration files were in .js.
I'd like to switch to .ts. The thing is, I cannot have both .js and .ts at the same time (from what I tried).

Describe the solution you'd like

Ideally, I'd like to be able to have a mix of .js and .ts migration files.
Maybe by specifying something in this option migrationFileExtension: ...

Describe alternatives you've considered

I could in theory switch all the files extensions to .ts even for past migrations, but then I would need to change the changelog fileName for past migration, and I'm pretty uncomfortable with this.