feat(function-lint): add Self-return patterns and MustNotExist rule for builder-style APIs

[scottgerring]

This PR introduces three new ReturnTypePatterns (SelfValue, SelfRef, SelfMutRef) and the MustNotExist rule. This lets us do things like assert how builders should look:

builder
    .function_lint()
      .lint_named("builder_style_with_consuming_forbidden")
      // forbid any `with_*` method that returns `Self`
      .matching(|m| m.name_regex("^with_").and(m.returns_self()))
      .with_severity(Severity::Error)
      .must_not_exist()
      .build();

The rule forces code to follow the consuming-builder pattern:

impl WidgetBuilder {
    pub fn set_height(&mut self, h: u32) -> &mut Self {
        self.height = h;
        self
    }
}

and not this:

impl WidgetBuilder {
    pub fn with_height(self, h: u32) -> Self { /* ... */ } 
}```