Add --system-root parameter, systemRoot() and path() functions to dsc

[SteveL-MSFT]

PR Summary

To enable offline configuration management, add --system-root parameter and systemRoot() function (takes no arguments) to DSC. The provided path must exist. Supported resources would need to accept the path to use rather than the current running OS.
If --system-root is not specified, then systemRoot() will default to $env:SYSTEMDRIVE on Windows and / on Linux/macOS.
Also added path() function that takes any number of strings and concatenates it with the OS directory separator char.

PR Context

Fix #534

[SteveL-MSFT]

Thinking about this further, I'm going to propose a few changes:

• rename parameter --mounted-path to --target-path to make it more general
• validate value of --target-path as existing otherwise error
• don't have targetPath() accept any arguments
• add new path(a, b, c) function that takes arbitrary number of parameters and just concatenates them with directory separator appropriate to the OS it's running on in between

The use of targetPath() should indicate that the configuration can work offline and online where it's an empty string if not provided via --target-path. The new path() function would enable use like [path(env('HOME'), '.data')] or [path(targetPath(), 'Windows', 'System32')].

Although for the latter case to work online, targetPath() would need to return the current OS drive on Windows like C: and I suppose on non-Windows it would simply be /.

[tgauth]

Would it be helpful to have some sort of info logging (maybe in the parser?) in the case of --target-path being provided without TargetPath() or is it obvious that --target-path would be ignored?

[SteveL-MSFT]

Would it be helpful to have some sort of info logging (maybe in the parser?) in the case of --target-path being provided without TargetPath() or is it obvious that --target-path would be ignored?

That could be useful as a warning message if the user provided --target-path but the configuration doesn't use it. The challenge is that expressions are parsed at runtime, so the warning would only happen at the end after configuration was run. I think this is ok.

Creatd #591 for future static linting work

[SteveL-MSFT]

Decided to rename to systemRoot instead of targetPath as it more accurately describes what it does