diagnostics not cleared when file is closed

The extension doesn't clear the editor diagnostics for a specific file when the file is closed, and diagnostics are persisted until the entire editor is closed, or the window reloaded.

For false-positives/non-issues like the one reported in #375, that means that whenever I open a workflow/action `.yml` file, the "Problems" pane is populated with non-actionable diagnostics, and I have no way to clear it unless I close the `.yml` file and reload the editor or the extension host.

**Expected behavior**:
The extension clears all diagnostics related to a file when it is closed, and only display diagnostics for actively opened files.

Inizia tracciando il modo in cui l’estensione crea e aggiorna i diagnostici per i file .yml di workflow/action aperti, quindi riproduci il problema aprendone e chiudendone uno. Il lavoro è completato quando i diagnostici di un file chiuso vengono cancellati, mentre i diagnostici dei file attivamente aperti rimangono visibili.

Scritto dal modello di indicizzazione a partire dal testo della issue.