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/main.lux

@@ -0,0 +1,20 @@
+// Main program that imports modules
+import examples/modules/math_utils
+import examples/modules/string_utils
+
+fn main(): Unit with {Console} = {
+    Console.print("=== Testing Module Imports ===")
+
+    // Use math_utils
+    Console.print("square(5) = " + toString(math_utils.square(5)))
+    Console.print("cube(3) = " + toString(math_utils.cube(3)))
+    Console.print("factorial(6) = " + toString(math_utils.factorial(6)))
+    Console.print("sumRange(1, 10) = " + toString(math_utils.sumRange(1, 10)))
+
+    // Use string_utils
+    Console.print(string_utils.greet("World"))
+    Console.print(string_utils.exclaim("Modules work"))
+    Console.print("repeat(\"ab\", 3) = " + string_utils.repeat("ab", 3))
+}
+
+let output = run main() with {}