This commit adds support for starting debug sessions through special
`vscode://llvm-vs-code-extensions.lldb-dap/start?config={launch-config}`
URIs. This allows tighter integration with custom scripts. One potential
use case is providing similar functionality to `xcdebug`, see #125777
for some discussion on that use case.
The functionality was inspired by @vadimcn's CodeLLDB extension, which
[provides similar
functionality](https://github.com/vadimcn/codelldb/blob/master/MANUAL.md#debugging-externally-launched-code).
