Use DAP's 'startDebugging' request

[weinand]

In order to make js-debug easier to use in other DAP clients, we should try to move VS Code specific "integration particularities" (typically extension code), into the debug adapter itself.

Using the new startDebugging reverse request should make this easier.

For details see: microsoft/debug-adapter-protocol#79

[connor4312]

Depends in part on microsoft/vscode#160169 for testing

[connor4312]

I started this on the dap-server branch, that adds a new standalone pure-DAP server. Still some rough edges though.

[filipdutescu]

will you continue work on it when you get more time, @connor4312, or is it discontinued for now? I would love to use it when available!

[icholy]

@filipdutescu I'm sure @connor4312 would appreciate feedback on the dap-server branch he pushed. Maybe try hooking it up to your editor and see if it works?