Debugger watchpoints for variable values

Add debugger watchpoints so an ObjoBasic debug session can pause automatically when a variable reaches a configured value. This would let developers track down state changes without stepping through large loops or adding temporary logging.

Initial user-facing behaviour could allow a developer to choose a variable in scope, enter an expected value, and have Studio break as soon as the variable matches that value during execution. The debugger should show which watchpoint fired and the current variable value.