Bind Shift-Backspace in the base keymap

FIX: Add a binding for Shift-Backspace to the base keymap, so that shift or caps-lock
won't interfere with backspace behavior.

See https://discuss.prosemirror.net/t/need-to-backspace-twice-to-remove-empty-paragraph-on-ios/4065

Author: marijnh

src/commands.js

@@ -609,6 +609,7 @@ export let pcBaseKeymap = {
   "Mod-Enter": exitCode,
   "Backspace": backspace,
   "Mod-Backspace": backspace,
+  "Shift-Backspace": backspace,
   "Delete": del,
   "Mod-Delete": del,
   "Mod-a": selectAll