feat: don't attach debugger to terminal until a process starts

[connor4312]

As referenced in #199, the debugger terminal experience today is awkward
in that a session is 'running' as long as the terminal is open, even
when no Node.js processes are currently active. This PR addresses that.

We reuse the TerminalNodeLauncher to set up the terminal, but do so
outside of the debug adapter. Instead, when we see a process launches
and enters debug mode, we register it inside of a 'delegate' collection,
and then send the delegate ID into vscode.debug.startDebugging. This
pulls up the debug session as if it was just launched insider the
adapter, and from then on it's identical to any other debug session.

This PR also adds an configuration option that configures the default
launch config used for the debugger terminal, and there's churn from
typing the targetOrigin away from any throughout the process, now
that I've fully internalized what it is and what it's used for :)

This implementation has some indirection, but within the adapter's
architecture I believe this is the best general approach. Due to the way
in which we launch processes and the way the VS Code terminal behaves
(both of which are optimal), we can't ever know that we have a
debuggable target exists until it knocks on our door with a CDP
connection. At that point, we need to delegate it into a debug session
in some form or another. That's what I've tried to do here with a
minimal amount of changes or knowledge-leaks into the rest of the code.

Fixes #199

[roblourens]

This is interesting but what's the point of declaring the types here?

[connor4312]

When dispatching commands between parts of the extension I wanted a way to make sure my types were done up correctly; refactoring some of the commands made me wish that TS could verify my work, so I added these.