fix(compiler): type-checking error for duplicate variables in templates (#35674)

It's an error to declare a variable twice on a specific template:

```html
<div *ngFor="let i of items; let i = index">
</div>
```

This commit introduces a template type-checking error which helps to detect
and diagnose this problem.

Fixes #35186

PR Close #35674

tools/public_api_guard/error_code.d.ts

@@ -30,5 +30,6 @@ export declare enum ErrorCode {
     MISSING_REFERENCE_TARGET = 8003,
     MISSING_PIPE = 8004,
     WRITE_TO_READ_ONLY_VARIABLE = 8005,
+    DUPLICATE_VARIABLE_DECLARATION = 8006,
     INJECTABLE_DUPLICATE_PROV = 9001
 }