Support tuple targets in for loops

[Green00101]

Closes #455

Summary

This PR adds support for tuple targets in for loops.

Example:

for a, b in pairs:
    pass

Before this change, for loops only supported a single name target.

Changes

• allow ast.For.target to be either a single name or a list of names
• parse tuple targets in for loops
• declare and flatten all names from tuple targets
• emit UnpackAssign in _desugar_For for tuple targets
• update backend output to print for a, b in pairs:

Notes

This PR supports simple tuple targets only.
Nested unpacking is still rejected.

Testing

Added tests for parser and backend output.

While testing, I found a separate doppler issue:
for t in lst fails when lst is list[tuple[...]], with what seems to be a gc_ref[tuple] vs tuple mismatch.

Because that looks independent from tuple targets in for loops, I did not include that compiler-level test in this PR.

Happy to make any changes if needed.

[antocuni]

Hello @Green00101.
Before I review, could you please disclose whether you used any AI assistant and, in case, how much?

[Green00101]

Sure.

I first used Opus to understand the codebase and identify the key functions I should focus on. Then, I used ChatGPT to help me understand how those important functions work.

Next, I tried to add code to the codebase by myself. After that, I used ChatGPT to review my code and make changes based on its feedback.

Finally, I used Opus to generate test cases. During this process, I encountered a strange failure that I could not understand. I asked three different AI tools about it, and they all gave me the same answer: it was a Doppler issue. So I mentioned that in the description.