feat(cpu): implement format 18 THUMB instructions

src/cpu/thumb/format18.zig

@@ -0,0 +1,15 @@
+const std = @import("std");
+
+const Bus = @import("../../Bus.zig");
+const Arm7tdmi = @import("../../cpu.zig").Arm7tdmi;
+const InstrFn = @import("../../cpu.zig").ThumbInstrFn;
+const u32SignExtend = @import("../../util.zig").u32SignExtend;
+
+pub fn format18() InstrFn {
+    return struct {
+        fn inner(cpu: *Arm7tdmi, _: *Bus, opcode: u16) void {
+            const offset = u32SignExtend(11, opcode & 0x7FF) << 1;
+            cpu.r[15] = (cpu.r[15] + 2) +% offset;
+        }
+    }.inner;
+}