[flake8-bugbear] Implement mutable-contextvar-default (B039) (#12113)

## Summary

Implement mutable-contextvar-default (B039) which was added to
flake8-bugbear in https://github.com/PyCQA/flake8-bugbear/pull/476.

This rule is similar to [mutable-argument-default
(B006)](https://docs.astral.sh/ruff/rules/mutable-argument-default) and
[function-call-in-default-argument
(B008)](https://docs.astral.sh/ruff/rules/function-call-in-default-argument),
except that it checks the `default` keyword argument to
`contextvars.ContextVar`.

```
B039.py:19:26: B039 Do not use mutable data structures for ContextVar defaults
   |
18 | # Bad
19 | ContextVar("cv", default=[])
   |                          ^^ B039
20 | ContextVar("cv", default={})
21 | ContextVar("cv", default=list())
   |
   = help: Replace with `None`; initialize with `.set()` after checking for `None`
```

In the upstream flake8-plugin, this rule is written expressly as a
corollary to B008 and shares much of its logic. Likewise, this
implementation reuses the logic of the Ruff implementation of B008,
namely


f765d19402/crates/ruff_linter/src/rules/flake8_bugbear/rules/function_call_in_argument_default.rs (L104-L106)

and 


f765d19402/crates/ruff_linter/src/rules/flake8_bugbear/rules/mutable_argument_default.rs (L106)

Thus, this rule deliberately replicates B006's and B008's heuristics.
For example, this rule assumes that all functions are mutable unless
otherwise qualified. If improvements are to be made to B039 heuristics,
they should probably be made to B006 and B008 as well (whilst trying to
match the upstream implementation).

This rule does not have an autofix as it is unknown where the ContextVar
next used (and it might not be within the same file).

Closes #12054

## Test Plan

`cargo nextest run`

crates/ruff_python_semantic/src/model.rs

@@ -1233,6 +1233,7 @@ impl<'a> SemanticModel<'a> {
             "_typeshed" => self.seen.insert(Modules::TYPESHED),
             "builtins" => self.seen.insert(Modules::BUILTINS),
             "collections" => self.seen.insert(Modules::COLLECTIONS),
+            "contextvars" => self.seen.insert(Modules::CONTEXTVARS),
             "dataclasses" => self.seen.insert(Modules::DATACLASSES),
             "datetime" => self.seen.insert(Modules::DATETIME),
             "django" => self.seen.insert(Modules::DJANGO),
@@ -1820,6 +1821,7 @@ bitflags! {
         const TYPESHED = 1 << 16;
         const DATACLASSES = 1 << 17;
         const BUILTINS = 1 << 18;
+        const CONTEXTVARS = 1 << 19;
     }
 }