Add Debugger

[lukexor]

An older version of tetanes had a basic debugger allowing you to step through CPU instructions, set breakpoints, and displayed info about the internal CPU state. This functionality was removed when the UI was overhauled to use egui.

In the Debug menu, Step Into, Step scanline, and Step Frame are currently implemented but without additional UI detail, they're relatively useless.

Desired initial MVP features:

• Ability to modify CPU registers
• Hex editor for modifying ROM/RAM
• Support for labeling memory addresses which persist across sessions
• Disassembly of CPU instructions
• IRQ/NMI state
• Ability to modify input states per controller
• Instruction/memory breakpoints for Read/Write/Execute (with ability to ignore dummy reads/writes)
• Step Into
• Step Out - should only step out within a function (e.g. the stack is empty), otherwise falls back to Step Into
• Step Over - Should only step over if the next instr is a jmp, otherwise falls back to Step Into
• Step Scanline
• Step Frame

Inspiration: https://www.mesen.ca/docs/debugging/debugger.html