feat: implement generic type parameters

Add full support for user-defined generic types and functions:

- Add type_params field to TypeChecker to track type parameters in scope
- Update resolve_type() to resolve type parameters to their bound variables
- Update function_type() to bind type parameters and return polymorphic TypeScheme
- Update type declaration handling for generic ADTs (e.g., Pair<A, B>)

Generic functions and types now work:
  fn identity<T>(x: T): T = x
  type Pair<A, B> = | MkPair(A, B)
  fn first<A, B>(p: Pair<A, B>): A = ...

Add examples/generics.lux demonstrating:
- Generic identity function
- Generic Pair type with first/second accessors
- Generic mapOption function

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

examples/generics.lux

@@ -0,0 +1,61 @@
+// Demonstrating generic type parameters in Lux
+//
+// Expected output:
+// identity(42) = 42
+// identity("hello") = hello
+// first(MkPair(1, "one")) = 1
+// second(MkPair(1, "one")) = one
+// map(Some(21), double) = Some(42)
+
+// Generic identity function
+fn identity<T>(x: T): T = x
+
+// Generic pair type
+type Pair<A, B> =
+    | MkPair(A, B)
+
+fn first<A, B>(p: Pair<A, B>): A =
+    match p {
+        MkPair(a, _) => a
+    }
+
+fn second<A, B>(p: Pair<A, B>): B =
+    match p {
+        MkPair(_, b) => b
+    }
+
+// Generic map function for Option
+fn mapOption<T, U>(opt: Option<T>, f: fn(T): U): Option<U> =
+    match opt {
+        None => None,
+        Some(x) => Some(f(x))
+    }
+
+// Helper function for testing
+fn double(x: Int): Int = x * 2
+
+// Test usage
+let id_int = identity(42)
+let id_str = identity("hello")
+
+let pair = MkPair(1, "one")
+let fst = first(pair)
+let snd = second(pair)
+
+let doubled = mapOption(Some(21), double)
+
+fn showOption(opt: Option<Int>): String =
+    match opt {
+        None => "None",
+        Some(x) => "Some(" + toString(x) + ")"
+    }
+
+fn printResults(): Unit with {Console} = {
+    Console.print("identity(42) = " + toString(id_int))
+    Console.print("identity(\"hello\") = " + id_str)
+    Console.print("first(MkPair(1, \"one\")) = " + toString(fst))
+    Console.print("second(MkPair(1, \"one\")) = " + snd)
+    Console.print("map(Some(21), double) = " + showOption(doubled))
+}
+
+let output = run printResults() with {}