Reapply "[Dexter] Add DAP support for Dexter, including lldb-dap (llvm#149394)"

This reverts commit e1e312e.

The original patch had some python3.10 syntax, which was incompatible with
the python3.8 minimum version in LLVM currently; this reapply brings
everything down to 3.8 compatibility. Separately, some incorrect arguments
were passed in LLDBDAP._get_launch_params, which have been removed.

Author: SLTozer