VS Code has a shared flag for extensions to use (env.isTelemetryEnabled) so that each extension doesn't do its own prompting.

We should switch to this, but still honour any explicit opt-outs from dart.allowAnalytics, but hide that setting and update any docs about it to point to the VS Code switch.

https://code.visualstudio.com/api/references/vscode-api#env.isTelemetryEnabled