[Feature] Allow env_add_path and shortcuts field to use variables

[abgox]

Feature Request

"env_add_path": [
    "$home\\.local\\bin",
    "$env:ProgramFiles\\Git\\bin"
],
"shortcuts": [
    [
        "$env:ProgramFiles\\Git\\git.exe",
        "Git"
    ]
]

• I hope scoop can automatically parse these variables.
• It does not affect the original functions of scoop, but can greatly improve the flexibility of the manifest.
• We can use $ExecutionContext.InvokeCommand.ExpandString()