LibGUI: Add Ctrl-W to insert mode

In Vim's insert mode, Ctrl-W deletes the word before the cursor, like
Ctrl-Backspace. Unlike Ctrl-Backspace, if only whitespace exists between
the end of the word and the cursor, the word will be deleted with the
whitespace.

To do so, this commit introduces two methods: delete_previous_word() for
TextEditor and first_word_before() for TextDocument, where the former
depends on the latter. delete_previous_word() is then called in
VimEditingEngine.

Userland/Libraries/LibGUI/TextEditor.h

@@ -135,6 +135,7 @@ public:
     void paste();
     void do_delete();
     void delete_current_line();
+    void delete_previous_word();
     void select_all();
     virtual void undo();
     virtual void redo();