LibGUI: Add Ctrl-H to insert mode

In Vim, Ctrl-H is effectively equivalent to backspace in insert mode, as
it deletes the previous character.

This commit implements method delete_previous_char() to TextEditor.
delete_char() already exists in EditingEngine, but it deletes the
next character rather than the previous. delete_previous_char() is then
called from within VimEditingEngine.

Userland/Libraries/LibGUI/TextEditor.cpp

@@ -911,6 +911,23 @@ void TextEditor::delete_current_line()
     execute<RemoveTextCommand>(document().text_in_range(erased_range), erased_range);
 }
 
+void TextEditor::delete_previous_char()
+{
+    if (!is_editable())
+        return;
+
+    if (has_selection())
+        return delete_selection();
+
+    TextRange to_erase({ m_cursor.line(), m_cursor.column() - 1 }, m_cursor);
+    if (m_cursor.column() == 0 && m_cursor.line() != 0) {
+        size_t prev_line_len = line(m_cursor.line() - 1).length();
+        to_erase.set_start({ m_cursor.line() - 1, prev_line_len });
+    }
+
+    execute<RemoveTextCommand>(document().text_in_range(to_erase), to_erase);
+}
+
 void TextEditor::do_delete()
 {
     if (!is_editable())

Userland/Libraries/LibGUI/TextEditor.h

@@ -136,6 +136,7 @@ public:
     void do_delete();
     void delete_current_line();
     void delete_previous_word();
+    void delete_previous_char();
     void select_all();
     virtual void undo();
     virtual void redo();

Userland/Libraries/LibGUI/VimEditingEngine.cpp

@@ -799,6 +799,9 @@ bool VimEditingEngine::on_key_in_insert_mode(const KeyEvent& event)
         case KeyCode::Key_W:
             m_editor->delete_previous_word();
             return true;
+        case KeyCode::Key_H:
+            m_editor->delete_previous_char();
+            return true;
         default:
             break;
         }