Allow the use of workspaceFolder without specifying workspaceMount

[Chuxel]

Today the workspaceFolder property requires workspaceMount to be in non-dockerCompose scenarios. Given processing for devcontainer.json should mount the location of the .git folder unless workspaceMount is set, it is reasonable to allow this property to be set independently.

In this scenario, the following syntax could be used to connect to a relative path to the detected auto-mount location.

"workspaceFolder": "${containerWorkspaceFolder}/some/sub/path" 

To avoid confusion, if workspaceMount is specified, or this is a Docker Compose scenario, ${containerWorkspaceFolder} would be treated like "". We could also introduce a different variable here instead (e.g. defaultContainerWorkspaceFolder) that had these same smarts in it.

[Santiagosaiz]

@Santiagosaiz4