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_not_num.wren

@@ -0,0 +1,3 @@
+import "timer" for Timer
+
+Timer.sleep("wat") // expect runtime error: Milliseconds must be a number.