feat: add specific error for attempting string[x] = ".." (#4611)

# Description

## Problem\*

Resolves #4267

## Summary\*

New error:

```bash
error: Strings do not support indexed assignment
  ┌─ /Users/michaelklein/Coding/rust/noir/test_programs/compile_failure/mutable_str_index/src/main.nr:3:5
  │
3 │     example[0] = "?";
  │     -------
  │

Aborting due to 1 previous error
```

## Additional Context



## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[Exceptional Case]** Documentation to be submitted in a separate
PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

compiler/noirc_frontend/src/hir/type_check/errors.rs

@@ -140,6 +140,8 @@ pub enum TypeCheckError {
         method_name: String,
         span: Span,
     },
+    #[error("Strings do not support indexed assignment")]
+    StringIndexAssign { span: Span },
 }
 
 impl TypeCheckError {
@@ -237,7 +239,8 @@ impl From<TypeCheckError> for Diagnostic {
             | TypeCheckError::ConstrainedReferenceToUnconstrained { span }
             | TypeCheckError::UnconstrainedReferenceToConstrained { span }
             | TypeCheckError::UnconstrainedSliceReturnToConstrained { span }
-            | TypeCheckError::NonConstantSliceLength { span } => {
+            | TypeCheckError::NonConstantSliceLength { span }
+            | TypeCheckError::StringIndexAssign { span } => {
                 Diagnostic::simple_error(error.to_string(), String::new(), span)
             }
             TypeCheckError::PublicReturnType { typ, span } => Diagnostic::simple_error(

compiler/noirc_frontend/src/hir/type_check/stmt.rs

@@ -258,6 +258,11 @@ impl<'interner> TypeChecker<'interner> {
                     Type::Array(_, elem_type) => *elem_type,
                     Type::Slice(elem_type) => *elem_type,
                     Type::Error => Type::Error,
+                    Type::String(_) => {
+                        let (_lvalue_name, lvalue_span) = self.get_lvalue_name_and_span(&lvalue);
+                        self.errors.push(TypeCheckError::StringIndexAssign { span: lvalue_span });
+                        Type::Error
+                    }
                     other => {
                         // TODO: Need a better span here
                         self.errors.push(TypeCheckError::TypeMismatch {