Add type-alias-readonlydeep rule and make ducks mostly immutable

.eslint/rules/type-alias-readonlydeep.js

@@ -0,0 +1,58 @@
+// Copyright 2023 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+function isReadOnlyDeep(node, scope) {
+  if (node.type !== 'TSTypeReference') {
+    return false;
+  }
+
+  let reference = scope.references.find(reference => {
+    return reference.identifier === node.typeName;
+  });
+
+  let variable = reference.resolved;
+  if (variable == null) {
+    return false;
+  }
+
+  let defs = variable.defs;
+  if (defs.length !== 1) {
+    return false;
+  }
+
+  let [def] = defs;
+
+  return (
+    def.type === 'ImportBinding' &&
+    def.parent.type === 'ImportDeclaration' &&
+    def.parent.source.type === 'Literal' &&
+    def.parent.source.value === 'type-fest'
+  );
+}
+
+/** @type {import("eslint").Rule.RuleModule} */
+module.exports = {
+  meta: {
+    type: 'problem',
+    hasSuggestions: false,
+    fixable: false,
+    schema: [],
+  },
+  create(context) {
+    return {
+      TSTypeAliasDeclaration(node) {
+        let scope = context.getScope(node);
+
+        if (isReadOnlyDeep(node.typeAnnotation, scope)) {
+          return;
+        }
+
+        context.report({
+          node: node.id,
+          message:
+            'Type aliases must be wrapped with ReadonlyDeep from type-fest',
+        });
+      },
+    };
+  },
+};