diagnostics not cleared when file is closed

[OmarTawfik]

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.