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/in_function_in_loop.wren

@@ -0,0 +1,7 @@
+var done = false
+while (!done) {
+  fn {
+    break // expect error
+  }
+  done = true
+}

test/break/in_method_in_loop.wren

@@ -0,0 +1,9 @@
+var done = false
+while (!done) {
+  class Foo {
+    method {
+      break // expect error
+    }
+  }
+  done = true
+}

test/break/in_while_loop.wren

@@ -0,0 +1,15 @@
+var i = 0
+while (true) {
+  i = i + 1
+  IO.write(i)
+  if (i > 2) {
+    // TODO: Should not require block for break.
+    break
+  }
+  IO.write(i)
+}
+// expect: 1
+// expect: 1
+// expect: 2
+// expect: 2
+// expect: 3

test/break/nested_loop.wren

@@ -0,0 +1,31 @@
+var i = 0
+while (true) {
+  IO.write("outer " + i.toString)
+  if (i > 1) {
+    // TODO: Should not require block for break.
+    break
+  }
+
+  var j = 0
+  while (true) {
+    IO.write("inner " + j.toString)
+    if (j > 1) {
+      // TODO: Should not require block for break.
+      break
+    }
+
+    j = j + 1
+  }
+
+  i = i + 1
+}
+
+// expect: outer 0
+// expect: inner 0
+// expect: inner 1
+// expect: inner 2
+// expect: outer 1
+// expect: inner 0
+// expect: inner 1
+// expect: inner 2
+// expect: outer 2

test/break/outside_loop.wren

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