Show SCXML parse errors in a Problems panel

[Phrogz]

Hand-editing XML can easily produce invalid markup. We know about this markup and log to the debug console in a stream of errors.

Instead, we should show errors in the Problems panel, and clear those errors when complete.

[Phrogz]

• https://stackoverflow.com/a/38308063/405017
• https://code.visualstudio.com/api/references/vscode-api#DiagnosticCollection