fix: fix for loop local variable detection

[dword-design]

Closes https://github.com/unjs/unimport/issues/487.

Generally, I was thinking that the regex solution feels very fragile. Parsing the file would probably the more stable solution, so there are probably more cases.

And we'd probably need a test and I'm not sure where to put it.

[antfu]

Could you help to add some tests for it? Thanks

[dword-design]

I added one. Doesn't look too bad.