Support file save triggered from the Firefox integrated version.

Related to https://bugzilla.mozilla.org/show_bug.cgi?id=1659753

This allows Firefox trigger a "save" event from ctrl/cmd+s or the "Save
Page As" context menu, which in turn lets pdf.js generate a new PDF if
there is form data to save.

I also now use `sourceEventType` on downloads so Firefox can determine if
it should launch the "open with" dialog or "save as" dialog.

src/display/api.js

@@ -2541,7 +2541,7 @@ class WorkerTransport {
         numPages: this._numPages,
         annotationStorage:
           (annotationStorage && annotationStorage.getAll()) || null,
-        filename: this._fullReader.filename,
+        filename: this._fullReader ? this._fullReader.filename : null,
       })
       .finally(() => {
         annotationStorage.resetModified();