Support definitions that reference intersystems.servers entries

[gjsjohnmurray]

This PR closes #8

1. Define a connection, picking "Server Definition" from the "Connect using" dropdown.
2. In the "Server name" field enter the name of the intersystems.servers server definition you want to use.
3. In the "Namespace" field enter the target namespace on that server.

If the intersystems.servers entry specifies a username this will be used. If no username property is defined you will be prompted at the time of first connection in a VS Code session.