Delete cell content works even when editable is set to false

[kemsky]

Expected behavior and actual behavior.

readonly grid, editable: false.

Steps to reproduce the problem.

select cell, press delete key, cell clears

This bug was introduced here: https://github.com/TonyGermaneri/canvas-datagrid/commit/439b3376f77b4d4c0ed4a7ffcc453c47deee9226

[kemsky]

            this.canvasDatagrid.addEventListener('keydown', (e) => {
                const event: KeyboardEvent = e.NativeEvent;
                if (getKeyCode(event) === 8 || getKeyCode(event) === 46) {
                    e.preventDefault();
                }
            });