Add #[mutants::exclude_re("pattern")] attribute

[sandersaares]

Add a new attribute that excludes specific mutations by regex, without disabling all mutations on the function like #[mutants::skip] does.

The attribute can be placed on functions, impl blocks, trait blocks, modules, and files (as an inner attribute). Patterns from outer scopes are inherited. Also supported within cfg_attr.

Changes

• mutants_attrs/src/lib.rs: New exclude_re proc-macro attribute (no-op, like skip)
• src/visit.rs: Core implementation - exclude_re_stack on the discovery visitor, push/pop at scope boundaries, attrs_exclude_re_patterns parsing, filtering in collect_mutant
• testdata/exclude_re_attr/: Test tree exercising all supported scopes (function, impl, trait, mod, scope inheritance, cfg_attr)
• tests/main.rs: Integration test
• book/src/attrs.md: Documentation
• NEWS.md: Changelog entry

Design decisions

• Invalid regex in an attribute returns a hard error, matching --exclude-re CLI behavior
• Multiple #[mutants::exclude_re] attributes on the same item are OR'd
• Patterns on outer scopes (impl/trait/mod/file) are inherited by all items within

Closes #551