Extract createAssetStringID

[Argmaster]

Extracted from #1207

[IntegratedQuantum]

This PR is doing too many things at once, making it difficult to review.
Please do the indentation changes in a later PR and only extract the helper function here.

Github is bad at displaying changes if you change the indentation and move code around. So please do them separately. (Or at least make separate commits, so I can look at them separately, but I've told you this already)

[Argmaster]

Here. Since the code was duplicated for readAllObjFilesInAddonsHashmap I replaced it there too.

[Argmaster]

Ok. I am refusing to figure out every possible edge case that is a result of this design decision.

[IntegratedQuantum]

All I said is that you should print an error if there is a dot before the extension (or only remove the extension, as it was before). Everything else is outside the scope of this PR anyways.

[Argmaster]

Here, for ".zig.zon" there is special case that removes it, for everything else there is generalized removal of last suffix, hopefully I didn't mess up the indexes. idk, they don't like me and I don't like them.

[IntegratedQuantum]

That's better, thank you.
(Sadly didn't save any lines, but we'll get there with the next read*Files function)