Add support for LSP DiagnosticRelatedInformation

[suoto]

If source A depends on source B, errors on source B should be included in the diagnostics shown for source A (warnings can be suppressed) via DiagnosticRelatedInformation (more info: https://microsoft.github.io/language-server-protocol/specifications/specification-3-14/#textDocument_publishDiagnostics).

This requires keeping track of the which of source A's dependencies triggered building source B (there might be more than one) so that the diagnostics from the latter can be included at a pertinent location on the former.

[pidgeon777]

This would be extremely cool and useful.