Add a discouraged feature for RegExp static properties (#2688)

The story of the specification is a little weird here, but I think this
tells an OK story to developers.

See-also: https://github.com/tc39/proposal-regexp-legacy-features/
See-also: tc39/ecma262#137

features/regexp-static-properties.yml

@@ -0,0 +1,13 @@
+name: RegExp static properties
+description: The `RegExp` object has several static properties to access the input and results of the most-recent regular expression match.
+spec: https://github.com/tc39/proposal-regexp-legacy-features
+discouraged:
+  according_to:
+    - https://github.com/tc39/proposal-regexp-legacy-features
+compat_features:
+  - javascript.builtins.RegExp.input
+  - javascript.builtins.RegExp.lastMatch
+  - javascript.builtins.RegExp.lastParen
+  - javascript.builtins.RegExp.leftContext
+  - javascript.builtins.RegExp.n
+  - javascript.builtins.RegExp.rightContext

features/regexp-static-properties.yml.dist

@@ -0,0 +1,40 @@
+# Generated from: regexp-static-properties.yml
+# Do not edit this file by hand. Edit the source file instead!
+
+status:
+  baseline: false
+  support:
+    chrome: "1"
+    chrome_android: "18"
+    edge: "12"
+    firefox: "1"
+    firefox_android: "4"
+    safari: "3"
+    safari_ios: "1"
+compat_features:
+  # baseline: false
+  # support:
+  #   chrome: "1"
+  #   chrome_android: "18"
+  #   edge: "12"
+  #   firefox: "1"
+  #   firefox_android: "4"
+  #   safari: "1"
+  #   safari_ios: "1"
+  - javascript.builtins.RegExp.n
+
+  # ⬇️ Same status as overall feature ⬇️
+  # baseline: false
+  # support:
+  #   chrome: "1"
+  #   chrome_android: "18"
+  #   edge: "12"
+  #   firefox: "1"
+  #   firefox_android: "4"
+  #   safari: "3"
+  #   safari_ios: "1"
+  - javascript.builtins.RegExp.input
+  - javascript.builtins.RegExp.lastMatch
+  - javascript.builtins.RegExp.lastParen
+  - javascript.builtins.RegExp.leftContext
+  - javascript.builtins.RegExp.rightContext

scripts/specs.ts

@@ -112,6 +112,10 @@ const defaultAllowlist: allowlistItem[] = [
     [
         "https://w3c.github.io/editing/docs/execCommand/",
         "Allowed because the execCommand feature points to it, to inform users that the feature is obsolete. The spec exists in a draft state only and will never move out of draft. It serves as a reference."
+    ],
+    [
+        "https://github.com/tc39/proposal-regexp-legacy-features",
+        "Allowed because it's the most spec-like thing that exists for discouraged RegExp static properties. Remove when https://github.com/tc39/ecma262/issues/137 is resolved."
     ]
 ];