feat: add module import examples and standard library

Module system was already implemented - this adds examples and stdlib:

examples/modules/:
- math_utils.lux: Reusable math functions (square, cube, factorial)
- string_utils.lux: String utilities (repeat, exclaim, greet)
- main.lux: Basic module import example
- main_selective.lux: Selective imports {fn1, fn2}
- main_wildcard.lux: Wildcard imports (module.*)
- use_stdlib.lux: Using the standard library

std/:
- prelude.lux: Core utilities (identity, compose, flip, not, and, or)
- io.lux: I/O helpers (println, readLine, debug)
- option.lux: Option utilities (some, none, map, flatMap, filter)
- result.lux: Result utilities (ok, err, mapOk, mapErr)

Import syntax supports:
- import path/to/module       (access as module.fn)
- import path/to/module as x  (access as x.fn)
- import path/to/module.{a,b} (access as a, b directly)
- import path/to/module.*     (all exports directly)

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

examples/modules/use_stdlib.lux

@@ -0,0 +1,23 @@
+// Example using the standard library
+import std/prelude.*
+import std/option as opt
+
+fn main(): Unit with {Console} = {
+    Console.print("=== Using Standard Library ===")
+
+    // Prelude functions
+    Console.print("identity(42) = " + toString(identity(42)))
+    Console.print("not(true) = " + toString(not(true)))
+    Console.print("and(true, false) = " + toString(and(true, false)))
+    Console.print("or(true, false) = " + toString(or(true, false)))
+
+    // Option utilities
+    let x = opt.some(10)
+    let y = opt.none()
+    Console.print("isSome(Some(10)) = " + toString(opt.isSome(x)))
+    Console.print("isNone(None) = " + toString(opt.isNone(y)))
+    Console.print("unwrapOr(Some(10), 0) = " + toString(opt.unwrapOr(x, 0)))
+    Console.print("unwrapOr(None, 0) = " + toString(opt.unwrapOr(y, 0)))
+}
+
+let output = run main() with {}