Implement a lint to find local variables created to allow this to be promoted #63888

Add a lint named unnecessary_this_alias, which will trigger if all the following conditions are met:

• The language feature this-promotion is enabled.
• A non-late local variable’s initializer is this.
• The variable’s static type (inferred or explicit) is the same as the type of this.
• The local variable is never written to.
• At least one usage of the local variable uses a syntax that would be expected to trigger type promotion (v is T, v as T, v == null, v != null, v!, etc.)