feat: add `filename` flag to change built filename

[MaximDevoir]

Adds a --filename (or -f) option to the NCC CLI to change the name of the emitted file from the standard index.c|js name.

fixes: #539, #492

[MaximDevoir]

Looks like there are some test errors. Unfortunately, I wasn't able to get testing working on a clean install of the repository. I'll look into getting tests to work locally so I can move forward with the PR.

[kohlerdominik]

would love to see this

[ttodua]

pity this PR was never merged