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/math_utils.lux

@@ -0,0 +1,14 @@
+// Math utilities module
+// Exports: square, cube, factorial
+
+pub fn square(n: Int): Int = n * n
+
+pub fn cube(n: Int): Int = n * n * n
+
+pub fn factorial(n: Int): Int =
+    if n <= 1 then 1
+    else n * factorial(n - 1)
+
+pub fn sumRange(start: Int, end: Int): Int =
+    if start > end then 0
+    else start + sumRange(start + 1, end)