Add missing unit opcode

Author: synapticvoid

src/bytecode/disassemble.zig

@@ -54,6 +54,7 @@ pub fn disassembleInstruction(writer: anytype, chunk: *const Chunk, offset: usiz
         .true,
         .false,
         .null,
+        .unit,
         .pop,
         .@"return",
         => {

src/bytecode/vm.zig

@@ -72,6 +72,7 @@ pub const Vm = struct {
                 .true => try self.push(.{ .boolean = true }),
                 .false => try self.push(.{ .boolean = false }),
                 .null => try self.push(.null),
+                .unit => try self.push(.unit),
                 .pop => _ = try self.pop(),
                 .negate => {
                     const val = try self.pop();