feat: add break statement support with loop tracking and error handling

Add TOKEN_BREAK keyword recognition and a loopBody field to the compiler
to track the innermost loop's first instruction index. The break
statement emits a jump to the end of the current loop, and compile-time
errors are reported when break appears outside a loop body, inside a
function defined within a loop, or inside a method defined within a
loop. New test files verify correct behavior for break in while loops,
nested loops, and error cases for invalid break usage.

test/break/outside_loop.wren

@@ -0,0 +1 @@
+break // expect error