ability to auto start a cmd (e.g. llama-server) with vscode

[emcodem]

Added a config item llama-vscode.launch_cmd that accepts a full cmd line as written in the shell/cmd prompt.
The Cmd is started on plugin activation and restarted whenever the launch_cmd config changes
When the Cmd exits, there is a toast message stating exit code and stderr

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• package.json: Language not supported

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src/llama-server.ts:81

• The launch command should be validated to ensure it is not an empty string before being used.

const launchCmd = this.extConfig.launch_cmd;

src/architect.ts:51

• [nitpick] The error message could be more user-friendly. Suggestion: The llama-vscode launch command process has closed with exit code ${data.code}. Error: ${data.stderr}.

vscode.window.showErrorMessage(`llama-vscode launchCmd Process closed, code: ${data.code}, stderr: ${data.stderr}`);