Add config update to auto-add a `"cwd": "${configDir}"` setting

[dsherret]

That way when someone does dprint config update it will automatically add that setting.

It's one of the methods on this handler: https://github.com/dprint/dprint-plugin-exec/blob/f0af32703966b0088c1fd1e4964d05f3b40ece81/src/handler.rs#L64