Support Relative Paths in Mandolin Settings

[wmccrthy]

This PR adds support for providing Mandolin configuration values (i.e foremanTomlPath, rulesPath, etc) as relative paths instead of absolute. This is a nice QoL feature that makes configuring Mandolin per-workspace easier and importantly, allows us to commit Mandolin settings to a repo once, for example, and have them work and apply to all users of that repo thereafter.

Paths are resolved relative to the workspace root of the open file when Mandolin runs.

[skberkeley]

Can you not just use ${workspaceFolder} in your settings?

[skberkeley]

Nevermind, looks like Microsoft only allows variables for certain privileged settings. I think we should still have users specify relative paths by using ${workspaceFolder}, as this seems to be the standard and just using a vanilla relative path could get messy. Can you update this to take the relative path using workspace.workspaceFolders instead?