[FEATURE] Allow using standard directories

[flying-sheep]

Is your feature request related to a problem? Please describe.
Same problem as in #229: some applications use OS standard directories as explained in the directories crate’s docs

Describe the solution you'd like

• If not already possible (didn’t see it in the docs) allow using variables in {settings,profiles.*}.dots.*.target
• pre-define variables for standard directories as exposed by the directories crate.

With this, the problem described in #229 would be as easy as:

[settings.dots]
vscode = { source = "editor/vscode.json", target = "%config_dir/Code/User/settings.json" }

Describe alternatives you've considered
#229 or a way to programmatically define profiles/variables