[GDScript] Cannot declare variable in lambda with the same name as parent scope

### Tested versions

Godot v4.3.stable - Windows 10.0.26100 - Vulkan (Forward+) - integrated Intel(R) Iris(R) Xe Graphics (Intel Corporation; 31.0.101.5595) - 13th Gen Intel(R) Core(TM) i5-1340P (16 Threads)

### System information

N/A

### Issue description

When creating a GDScript function containing a lambda in it, both with the same variable name, this will result in a compile error.

### Steps to reproduce

Create the following script below. It will result in the following error:

```
Parse Error: There is already a variable named "result" declared in this scope.
```

### Minimal reproduction project (MRP)

```gdscript
func test() -> void:
var result := []
var lambda := func () -> void:
var result := []
```

Start by running the minimal GDScript reproduction from the issue and confirming the scope error. Trace the GDScript compiler's handling of variable declarations in nested lambdas, then add or update a regression test for the shown case. Done means the script parses successfully while preserving errors for invalid duplicate declarations.

Written by the indexing model from the issue text.