refactor(compiler): implement let declarations in render3 ast (#55848)

Introduces a new `LetDeclaration` into the Render3 AST, simiarly to the HTML AST, and adds an initial integration into the various visitors.

PR Close #55848

packages/compiler-cli/src/ngtsc/indexer/src/template.ts

@@ -28,6 +28,7 @@ import {
   TmplAstForLoopBlockEmpty,
   TmplAstIfBlock,
   TmplAstIfBlockBranch,
+  TmplAstLetDeclaration,
   TmplAstNode,
   TmplAstRecursiveVisitor,
   TmplAstReference,
@@ -351,6 +352,10 @@ class TemplateVisitor extends TmplAstRecursiveVisitor {
     this.visitAll(block.children);
   }
 
+  override visitLetDeclaration(decl: TmplAstLetDeclaration): void {
+    this.visitExpression(decl.value);
+  }
+
   /** Creates an identifier for a template element or template node. */
   private elementOrTemplateToIdentifier(
     node: TmplAstElement | TmplAstTemplate,

packages/compiler-cli/src/ngtsc/typecheck/extended/api/api.ts

@@ -27,6 +27,7 @@ import {
   TmplAstIcu,
   TmplAstIfBlock,
   TmplAstIfBlockBranch,
+  TmplAstLetDeclaration,
   TmplAstNode,
   TmplAstRecursiveVisitor,
   TmplAstReference,
@@ -264,6 +265,10 @@ class TemplateVisitor<Code extends ErrorCode>
     this.visitAllNodes(block.children);
   }
 
+  visitLetDeclaration(decl: TmplAstLetDeclaration): void {
+    this.visitAst(decl.value);
+  }
+
   getDiagnostics(template: TmplAstNode[]): NgTemplateDiagnostic<Code>[] {
     this.diagnostics = [];
     this.visitAllNodes(template);