Add ability to capture device screenshots

[TwitchBronBron]

roku-deploy supports capturing screenshots since roku-deploy#92. Integrate this functionality into the VSCode extension.

Flow:

• Trigger "take a screenshot" by
  • Running a command from the command pallet
    • when triggering the command directly, we need to prompt the user to pick which Roku device to use
  • Clicking a button in the "Active Devices" panel
    • the user clicked the button on a specific device, so they don't need to pick a device
• the image is automatically saved to temp (that's the default way roku-deploy writes it right now)
• vscode will then open the image in a new tab in preview mode so the user can see it.

Considerations:
Here are some user settings that might make sense:

//should a preview window be shown after every screenshot
"brightscript.screenshots.enablePreview": true | false,

//What screeshot info should be copied to clipboard? The image data itself? The image path? or nothing?
"brightscript.screenshots.copyToClipboard": "image" | "path" | "disabled",

//where should the screenshots be placed? Defaults to the current system temp directory.
"brightscript.screenshot.imageDir": "${workspaceFolder}/.temp/images"

[TwitchBronBron]

This was implmeneted in #505