terminal: add support for opening external terminal windows

[vince-fugnitto]

Feature Description:

As an end-user, I'd like the possibility to open an external terminal through a new command and service similarly to vscode:

command: {
  id: OPEN_NATIVE_CONSOLE_COMMAND_ID,
  title: { value: nls.localize('globalConsoleAction', "Open New External Terminal"), original: 'Open New External Terminal' }
}

I believe the command would only make sense in an electron target.

Additional Details:

Documentation:

• https://code.visualstudio.com/docs/editor/integrated-terminal#:~:text=To%20open%20the%20terminal%3A,the%20View%20%3E%20Terminal%20menu%20command.

Implementation Details:

• https://github.com/microsoft/vscode/blob/master/src/vs/workbench/contrib/externalTerminal/common/externalTerminal.ts
• https://github.com/microsoft/vscode/blob/master/src/vs/workbench/contrib/externalTerminal/node/externalTerminalService.ts
• https://github.com/microsoft/vscode/blob/master/src/vs/workbench/contrib/externalTerminal/node/externalTerminal.contribution.ts

[vince-fugnitto]

I believe @dukengn would like to pick up the issue when he has time.