Use VS Code's env.isTelemetryEnabled flag as telemetry opt-in

[DanTup]

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

[DanTup]

The text for this setting suggests it's only for first party collection now, which doesn't seem to match the intention of env.isTelemetryEnabled 🤔

Controls all core and first party extension telemetry. This helps us to better understand how Visual Studio Code is performing, where improvements need to be made, and how features are being used. Read more about the data we collect and our privacy statement. A full restart of the application is necessary for crash reporting changes to take effect.

Note: If/when switching to this, we also need to either listen to env.onDidChangeTelemetryEnabled or ensure we read the latest value of env.isTelemetryEnabled at the point of sending the metrics.