Add ruff rule --all subcommand (with JSON output) (#5059)

## Summary

This adds a `ruff rule --all` switch that prints out a human-readable
Markdown or a machine-readable JSON document of the lint rules known to
Ruff.

I needed a machine-readable document of the rules [for a
project](https://github.com/astral-sh/ruff/discussions/5078), and
figured it could be useful for other people – or tooling! – to be able
to interrogate Ruff about its arcane knowledge.

The JSON output is an array of the same objects printed by `ruff rule
--format=json`.

## Test Plan

I ran `ruff rule --all --format=json`. I think more might be needed, but
maybe a snapshot test is overkill?

crates/ruff_cli/src/args.rs

@@ -35,11 +35,17 @@ pub struct Args {
 pub enum Command {
     /// Run Ruff on the given files or directories (default).
     Check(CheckArgs),
-    /// Explain a rule.
+    /// Explain a rule (or all rules).
     #[clap(alias = "--explain")]
+    #[command(group = clap::ArgGroup::new("selector").multiple(false).required(true))]
     Rule {
-        #[arg(value_parser=Rule::from_code)]
-        rule: Rule,
+        /// Rule to explain
+        #[arg(value_parser=Rule::from_code, group = "selector")]
+        rule: Option<Rule>,
+
+        /// Explain all rules
+        #[arg(long, conflicts_with = "rule", group = "selector")]
+        all: bool,
 
         /// Output format
         #[arg(long, value_enum, default_value = "text")]