Add break to the interpreter with a global break_flag (like return_flag). Add to the compiler as emit_op("br 1") -- unconditional exit from the block/loop pair. The VM already handles br 1.

    check_both(
        \\var i: i64 = 0;
        \\while (i < 100) {
        \\    if (i == 5) { break; }
        \\    i += 1;
        \\}
        \\i
    , 5);