Validate allowList and denyList before using them

[Tyriar]

Seeing this:

It's caused by using the WIP setting format:

  "github.copilot.chat.agent.terminal.allowList": {
    "echo": false,
    "Write-Host": false,
    "ls": true,
    "Get-ChildItem": true,
    "cat": true,
    "Get-Content": true,
    "pwd": true,
    "Get-Location": true,
    "cd": true,
    "Set-Location": true
  },

When it still expects an array in Insiders.