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 👍.