Implement inferior in its own terminal on Linux

[jonahgraham]

Using DAP's runInTerminal this PR adds the ability to use that new terminal for the inferior's I/O.

This can be enabled by adding "inferiorTerminal": "integrated" or "inferiorTerminal": "external" or "inferiorTerminal": "auto" to the launch.json.

The basic idea of the inferior terminal on Linux is:

• adapter requests client (aka vscode) to create a terminal (using runInTerminal)
• in that terminal we run a small script that "returns" the tty name to the adapter (using an atomically created file with the output of tty command)
• then the script waits until the adapter is complete by monitoring the PID of the adapter's node process

The script run in the terminal won't auto-stop when running the adapter in server mode (typically should only be used for development of the adapter)

Part of #161

TODO for this issue:

• [ ] write a test
• [ ] document the change here, or at least in cdt-gdb-vscode's package.json