fix: restore x/X/D as modal debug keys, add quit_exits_mode config, track mapped keys exactly

lua/tiny-debugger/help.lua

@@ -12,6 +12,9 @@ local modal_info = {
   { "step_into", "Step into" },
   { "step_out", "Step out" },
   { "run_to_cursor", "Run to cursor" },
+  { "breakpoint", "Toggle breakpoint" },
+  { "cond_breakpoint", "Conditional breakpoint" },
+  { "clear_breakpoints", "Clear all breakpoints" },
   { "watch", "Watch expression" },
   { "hover", "Hover / inspect" },
   { "eval", "Eval in REPL" },