Revert making Template.locals readonly

It turns out to be important that AST plugins can mutate the list
of local variables in the root `Template` node. Instead, use *that*
list of local variables as the ultimate source of truth after AST
plugin ran.

Author: chancancode

packages/@glimmer/syntax/lib/parser/tokenizer-event-handlers.ts

@@ -762,10 +762,6 @@ export function preprocess(
     options.locals ?? []
   );
 
-  if (options.strictMode && options.locals?.length) {
-    template = b.template({ ...template, locals: options.locals });
-  }
-
   if (options?.plugins?.ast) {
     for (const transform of options.plugins.ast) {
       let env: ASTPluginEnvironment = assign({}, options, { syntax }, { plugins: undefined });

packages/@glimmer/syntax/lib/v1/legacy-interop.ts

@@ -11,23 +11,24 @@ export function buildLegacyTemplate({ body, locals, loc }: TemplateParams): ASTv
   const node = {
     type: 'Template',
     body,
+    locals,
     loc,
   };
 
-  Object.defineProperty(node, 'locals', {
-    enumerable: true,
-    writable: false,
-    value: Object.freeze([...locals]),
-  });
-
   Object.defineProperty(node, 'blockParams', {
     enumerable: false,
-    get(): readonly string[] {
+    get(): string[] {
       deprecate(
         `Template nodes can never have block params, for in-scope variables, use locals instead`
       );
       return this.locals;
     },
+    set(value: string[]) {
+      deprecate(
+        `Template nodes can never have block params, for in-scope variables, use locals instead`
+      );
+      this.locals = value;
+    },
   });
 
   return node as ASTv1.Template;

packages/@glimmer/syntax/lib/v1/nodes-v1.ts

@@ -29,12 +29,12 @@ export type EntityEncodingState = 'transformed' | 'raw';
 
 export interface Template extends CommonProgram {
   type: 'Template';
-  readonly locals: readonly string[];
+  locals: string[];
 
   /**
    * @deprecated use locals instead
    */
-  readonly blockParams: readonly string[];
+  blockParams: string[];
 }
 
 /**

packages/@glimmer/syntax/lib/v2/normalize.ts

@@ -40,8 +40,8 @@ export function normalize(
 
   let normalizeOptions = {
     strictMode: false,
-    locals: [],
     ...options,
+    locals: ast.locals,
   };
 
   let top = SymbolTable.top(