feat: add input validation and stack trace filtering for Timer.sleep

Add type and range checks to Timer.sleep() in both C source and Wren
implementation, aborting with descriptive errors for non-numeric or
negative arguments. Introduce six new test files covering sleep with
fibers, float durations, main fiber, zero milliseconds, negative
values, and non-numeric input. Fix stack trace parsing in test runner
to skip builtin library frames by iterating through error lines until
a main-line match is found.

test/timer/sleep_main_fiber.wren

@@ -0,0 +1,9 @@
+import "timer" for Timer
+
+IO.print("1") // expect: 1
+Timer.sleep(3)
+IO.print("2") // expect: 2
+Timer.sleep(3)
+IO.print("3") // expect: 3
+Timer.sleep(3)
+IO.print("4") // expect: 4