Triple quoted strings cannot be compared to typical strings with the Expression class

### Tested versions

- Reproducible in v4.3.stable.artix_linux

### System information

Godot v4.3.stable unknown - Artix Linux

### Issue description

In the editor, if you compare a double quoted string to a triple double quoted string, they are equivalent.

If you use the Expression class to compare them, they are not equivalent.

I'm guessing it shouldn't matter whether it's a double, single, triple double or triple single quoted string. They should all be the same if the contents mean the same thing.

### Steps to reproduce

func evaluate():
if "x" == """x""":
print("sane")
else:
print("insane")

var xp = Expression.new()
var err = xp.parse('"x" == """x"""', [])
if err != OK:
push_error("failed to parse")
var res = xp.execute([])
if xp.has_execute_failed():
push_error(xp.get_error_text())
print('Does expression.execute() think they are the same?')
print(res)

Prints:

sane
Does expression.execute() think they are the same?
false

Should print:

sane
Does expression.execute() think they are the same?
true

For any combination of quote types.

### Minimal reproduction project (MRP)

[godot-quote-bug.zip](https://github.com/user-attachments/files/17501407/godot-quote-bug.zip)

Start with the Expression class's parse() and execute() entry points, using the attached godot-quote-bug.zip project to reproduce the discrepancy. Compare the handling of regular and triple-quoted strings in the provided expression. Done means Expression.execute() treats equivalent quote styles as equal, matching the editor result.

Written by the indexing model from the issue text.