feat: type aliasing with the alias keyword

[SchoolyB]

Summary

Add type aliasing to Grayscale using the alias keyword. An alias creates a
new name for an existing type — purely for readability. The alias and the
original type are fully interchangeable.

---

Syntax

alias PlayerID = int
alias Point3D = Point

Private aliases (scoped to the declaring file):

private alias ID = int

---

Rules

• File-scope only — no aliases inside functions, loops, or blocks
• Public by default, private restricts to the declaring file
• The alias and the underlying type are identical — fully interchangeable in all contexts
• Can alias any type: primitives, structs, enums, function signatures, containers
• No chained alias resolution issues — an alias of an alias just resolves to the base type

---

Compiler Changes

• Lexer: Register alias as a keyword
• Parser: Parse alias Name = Type as a new top-level declaration node
• Typechecker: Resolve alias references to their underlying type; validate the target type exists
• Codegen: Emit the underlying C type wherever the alias is used (aliases are erased at codegen)

---

Examples

alias Meters = float
alias Username = string

do calculate_distance(start Meters, end Meters) -> Meters {
    if end > start {
        return end - start
    }
    return start - end
}

do main() {
    mut distance Meters = calculate_distance(10.0, 25.5)
    println("Distance: ${distance}")
}

Since aliases are interchangeable, this also works:

mut distance float = calculate_distance(10.0, 25.5)

[SchoolyB]

completed