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/build.rs

@@ -0,0 +1,102 @@
+// Copyright 2018-2025 the Deno authors. MIT license.
+
+#![allow(clippy::disallowed_methods)]
+
+mod data;
+mod structs;
+
+fn main() {
+  let crate_dir = std::path::PathBuf::from(env!("CARGO_MANIFEST_DIR"));
+
+  let mut js_list = String::from(
+    "// Copyright 2018-2025 the Deno authors. MIT license.
+
+/**
+ * Don't modify this file manually.
+ *
+ * This file is auto-generated by the build script, modify `data.rs` instead.
+ */
+
+export const unstableIds = {
+",
+  );
+
+  let mut rs_list = String::from(
+    "// Copyright 2018-2025 the Deno authors. MIT license.
+
+/// Don't modify this file manually.
+///
+/// This file is auto-generated by the build script, modify `data.rs` instead.
+use crate::structs::UnstableFeatureDefinition;
+use crate::structs::UnstableFeatureKind;
+
+pub static UNSTABLE_FEATURES: &[UnstableFeatureDefinition] = &[",
+  );
+
+  let mut descriptions = data::FEATURE_DESCRIPTIONS.to_vec();
+  descriptions.sort_by_key(|desc| desc.name);
+
+  for (id, feature) in descriptions.into_iter().enumerate() {
+    let flag_name = format!("unstable-{}", feature.name);
+    let feature_kind = match feature.kind {
+      structs::UnstableFeatureKind::Cli => "UnstableFeatureKind::Cli",
+      structs::UnstableFeatureKind::Runtime => "UnstableFeatureKind::Runtime",
+    };
+    let env_var = match feature.env_var {
+      Some(var_name) => format!(r#"Some("{}")"#, var_name),
+      None => "None".to_string(),
+    };
+
+    rs_list += &format!(
+      r#"  UnstableFeatureDefinition {{
+    name: "{}",
+    flag_name: "{}",
+    help_text: "{}",
+    show_in_help: {},
+    id: {},
+    kind: {},
+    env_var: {},
+    config_file_option: "{}",
+  }},
+"#,
+      feature.name,
+      flag_name,
+      feature.help_text,
+      feature.show_in_help,
+      id,
+      feature_kind,
+      env_var,
+      match feature.config_option {
+        data::ConfigFileOption::SameAsFlagName => feature.name,
+        data::ConfigFileOption::Renamed(alias) => alias,
+      }
+    );
+
+    if matches!(feature.kind, structs::UnstableFeatureKind::Runtime) {
+      let camel = camel_case(feature.name);
+      js_list += &format!("  {}: {},\n", camel, id);
+    }
+  }
+
+  js_list += "};\n";
+  rs_list += "];\n";
+
+  std::fs::write(crate_dir.join("gen.js"), js_list).unwrap();
+  std::fs::write(crate_dir.join("gen.rs"), rs_list).unwrap();
+}
+
+fn camel_case(name: &str) -> String {
+  let mut output = String::new();
+  let mut upper = false;
+  for c in name.chars() {
+    if c == '-' {
+      upper = true;
+    } else if upper {
+      upper = false;
+      output.push(c.to_ascii_uppercase());
+    } else {
+      output.push(c);
+    }
+  }
+  output
+}