Resolve internal variables in formatter arguments

[zandivx]

Currently, VS Code variables like ${workspaceFolder} are not expanded in the arguments provided to the selected formatter. As this functionality is already implemented in this project and applied at several places, it's no effort to apply it to the arguments for the formatter as well.

[krystophny]

Thanks @zandivx ! This enables the use of a .fprettify file in the workspace folder

    "fortran.formatting.fprettifyArgs": [
        "--config",
        "${workspaceFolder}/.fprettify"
    ],

@gnikit this is a small change that is very useful, if you find time we would be very grateful if you could review this.