Compatibility mode for Source Depot

[hoovercj]

Addresses #47

The main issues were:

• Login isn't needed for Source Depot
• Source Depot doesn't support ztags
• Source Depot doesn't support the -T flag for fstat

[hoovercj]

• I haven't been able to test sync because my team doesn't actually use sd sync directly.
• Creating a new changelist from the commit box doesn't move the files from the default changelist. I don't think this is an important issue for sd, but please make sure I haven't broken that for perforce. I've tried it with and without the ztag flag and it made no difference, so I left the ztag flag in.

[stef-levesque]

Thanks! Nice work!

• I'll look to setContext, and remove logins and sync commands for now
• The current changelist workflow is not exactly how I want it to be. I'm still waiting on SCM: let SCMProvider knows when a SCMResourceGroup is selected microsoft/vscode#23594, SCM: Support drag-and-drop microsoft/vscode#23596, SCM: Tiptool or hover message for SCMResource and SCMResourceGroup microsoft/vscode#23599 but those will likely have to wait for the April release. In the meanwhile, moving all files in 'default' to the new changelist seems like a good workaround

[hoovercj]

regarding the default changelist, it is my understanding that the "change" command should do it automatically.

The sync command shouldn't be removed. We have an "sd sync" and it is a valid command, it's just that my team doesn't use it directly so I can't test it. Please leave that in for others.

Hiding the login behind a setContext might work. I don't know why the config check wasn't working, maybe you can figure it out, otherwise we should open a vscode issue to get to the bottom of it.