Allow .bs.mjs extension

[wokalski]

This has been mentioned a couple of time. Not having this causes serious issues if both Typescript and rescript are present in a codebase and there exist modules with the same name .res and .ts in the same directory.

Follows: #4832, #4116

.mjs support was implemented but .bs.mjs was not.

[tsnobip]

closed by #5631