Add new rule RUF059: Unused unpacked assignment (#16449)

Split from F841 following discussion in #8884.

Fixes #8884.

<!--
Thank you for contributing to Ruff! To help us out with reviewing,
please consider the following:

- Does this pull request include a summary of the change? (See below.)
- Does this pull request include a descriptive title?
- Does this pull request include references to any relevant issues?
-->

## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

Add a new rule for unused assignments in tuples. Remove similar behavior
from F841.

## Test Plan

Adapt F841 tests and move them over to the new rule.

<!-- How was it tested? -->

---------

Co-authored-by: Micha Reiser <micha@reiser.io>

crates/ruff_server/src/lint.rs

@@ -445,7 +445,8 @@ fn tags(code: &str) -> Option<Vec<lsp_types::DiagnosticTag>> {
     match code {
         // F401: <module> imported but unused
         // F841: local variable <name> is assigned to but never used
-        "F401" | "F841" => Some(vec![lsp_types::DiagnosticTag::UNNECESSARY]),
+        // RUF059: Unused unpacked variable
+        "F401" | "F841" | "RUF059" => Some(vec![lsp_types::DiagnosticTag::UNNECESSARY]),
         _ => None,
     }
 }