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.

Empieza trazando cómo la extensión crea y actualiza los diagnósticos de los archivos .yml de workflow/action abiertos, y luego reproduce el problema abriendo y cerrando uno. Se considera terminado cuando se borran los diagnósticos de un archivo cerrado, mientras los diagnósticos de los archivos abiertos activamente siguen visibles.

Escrito por el modelo de indexación a partir del texto del issue.