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.

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(