feat: Codegen feature flags (#28920)

This commit adds "deno_features" crate that contains definitions of all
unstable features in Deno.

Based on these definitions, both Rust and JS code is generated ensuring
that the two are always in sync.

In addition some of flag handling was rewritten to use the generated
definitions, instead of hand rolling these flag definitions.

---------

Co-authored-by: snek <snek@deno.com>

runtime/features/structs.rs

@@ -0,0 +1,20 @@
+// Copyright 2018-2025 the Deno authors. MIT license.
+
+#[derive(Clone, Debug)]
+pub enum UnstableFeatureKind {
+  Cli,
+  Runtime,
+}
+
+#[derive(Debug)]
+#[allow(dead_code)]
+pub struct UnstableFeatureDefinition {
+  pub name: &'static str,
+  pub flag_name: &'static str,
+  pub help_text: &'static str,
+  pub show_in_help: bool,
+  pub id: i32,
+  pub kind: UnstableFeatureKind,
+  pub env_var: Option<&'static str>,
+  pub config_file_option: &'static str,
+}