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

This patch adds a new variety of driver to Dexter, allowing it to work
with DAP-based interfaces for debuggers. The first concrete instance of
this is implemented in this patch, adding support for an `lldb-dap`
debugger. This is functionally very similar to the existing LLDB
debugger support*, but uses lldb-dap as its executable instead of lldb.

This has been tested successfully against the existing feature_test
suite, and manually tested against some other inputs; support is
essentially complete, although any further DAP-based debuggers may
require additional hooks inserted into the base class to deal with any
idiosyncrasies they exhibit (as with the several that have been inserted
for lldb-dap).

NB: There are some small differences resulting from differences between
lldb-dap's use of the lldb API and Dexter's use in its lldb driver; one
small example of this is when evaluating variables, lldb-dap will try to
first use `GetValueForVariablePath` and fallback to `EvaluateExpression`
if necessary, while Dexter will always use `EvaluateExpression`; these
can give slightly different results, resulting in different output from
Dexter for the same input.

Author: SLTozer