feat: implement out inline declaration syntax (out value: type)

[antonsynd]

Context

The parameter_modifiers.md spec documents out value: int inline declaration syntax at call sites, but it is not yet implemented.

Example:

from system import Int32
if Int32.try_parse("42", out value: int):
    print(value)

Should emit:

if (Int32.TryParse("42", out int value))
{
    Console.WriteLine(value);
}

What's needed

• Parser: extend ModifiedArgument parsing to handle out identifier: type pattern
• Semantic: register the new variable in scope, validate type
• CodeGen: emit out type identifier in C# argument position

Related

• Implement struct parameter modifiers (in[], mut[], out[]) #419 — parent issue for parameter modifier syntax (ref/out/in basics are done)
• parameter_modifiers.md spec section "Inline Declaration with out"