Structure code for future LLDB support

[Baltoli]

This PR sets up the infrastructure for future K support of LLDB (#701), but doesn't yet implement the K-specific debugging features. Future PRs can follow on implementing individual debugger support features by hooking into the dispatch_k function.

The stubbed LLDB plugin as implemented here just attaches to the interpreter, sets a breakpoint before main, then drops into the default LLDB REPL. Any K commands (start, step etc.) just produce a warning.

I'll follow this up with a corresponding frontend PR (similar to runtimeverification/k#2646) that launches klldb.py with the appropriate arguments when --debugger is passed to krun on macOS, and updates the documentation accordingly.

[dwightguth]

This looks fine as far as it goes, so I'm approving, but if we name the lldb script klldb.py, should we rename the gdb script kgdb.py?

[Baltoli]

Good point - I'll do that then merge if you're happy.

[dwightguth]

Yes, that's fine. That's why I approved.