refactor: Generate Linter -> RuleSelector mapping via macro

To enable ruff_dev to automatically generate the rule Markdown tables in
the README the ruff library contained the following function:

    Linter::codes() -> Vec<RuleSelector>

which was slightly changed to `fn prefixes(&self) -> Prefixes` in
9dc66b5a65 to enable ruff_dev to split
up the Markdown tables for linters that have multiple prefixes
(pycodestyle has E & W, Pylint has PLC, PLE, PLR & PLW).

The definition of this method was however largely redundant with the
#[prefix] macro attributes in the Linter enum, which are used to
derive the Linter::parse_code function, used by the --explain command.

This commit removes the redundant Linter::prefixes by introducing a
same-named method with a different signature to the RuleNamespace trait:

     fn prefixes(&self) -> &'static [&'static str];

As well as implementing IntoIterator<Rule> for &Linter. We extend the
extisting RuleNamespace proc macro to automatically derive both
implementations from the Linter enum definition.

To support the previously mentioned Markdown table splitting we
introduce a very simple hand-written method to the Linter impl:

    fn categories(&self) -> Option<&'static [LinterCategory]>;

scripts/add_plugin.py

@@ -89,12 +89,6 @@ mod tests {
                 fp.write(f"{indent}{pascal_case(plugin)},")
                 fp.write("\n")
 
-            elif line.strip() == "Linter::Ruff => Prefixes::Single(RuleSelector::RUF),":
-                fp.write(
-                    f"{indent}Linter::{pascal_case(plugin)} => Prefixes::Single(RuleSelector::{prefix_code}),"
-                )
-                fp.write("\n")
-
             fp.write(line)
             fp.write("\n")