Support environment variables inside shellPath

[Ash258]

I would like to be able to setup shellpath using ${env:SOMEKEY}\\executable.exe same as in tasks.json.

My settings are synced across multiple devices and on one machine i have git bash in C drive and on other machine in S drive.

So instead of this:

{
    "terminals.terminals": [
        {
            "name": "Git Bash",
            "shellPath": "C:\\SCOOP\\apps\\git\\current\\bin\\bash.exe",
            "focus": true,
            "open": true,
        }
}

i would need to setup it like this:

{
    "terminals.terminals": [
        {
            "name": "Git Bash",
            "shellPath": "${env:GIT_INSTALL_ROOT}\\bin\\bash.exe",
            "focus": true,
            "open": true,
        }
}

[fabiospampinato]

PR welcome for implementing the [env:*] token 👍.