Created the accessibility help dialog. (#15702)

Feature (ui): Implemented the `AccessibilityHelp` plugin that brings a dialog displaying keyboard shortcuts available in the editor. Closes #1014.

Feature (core): Brought the `editor.accessibility` namespace to the base `Editor` class as a container for accessibility-related features and systems. See #1014.

Other (essentials): Enabled the `AccessibilityHelp` plugin by default. See #1014.

Fix (utils): The exported `keyCodes` object should contain correct codes for keys related to punctuation, brackets, braces, etc. See #1014.

docs/_snippets/features/keyboard-support.html

@@ -0,0 +1,6 @@
+<!-- include CKBox from CDN -->
+<script src="https://cdn.ckbox.io/ckbox/latest/ckbox.js"></script>
+
+<div id="keyboard-support">
+	<p>Press <kbd>Alt</kbd>+<kbd>0</kbd> (<kbd>⌥0</kbd> on Mac) while editing to display the list of available keyboard shortcuts.</p>
+</div>