Storybook: Add stories for HeadingLevelDropdown component

packages/block-editor/src/components/block-heading-level-dropdown/stories/index.story.js

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+/**
+ * WordPress dependencies
+ */
+import { useState } from '@wordpress/element';
+
+/**
+ * Internal dependencies
+ */
+import HeadingLevelDropdown from '../';
+
+const meta = {
+	title: 'BlockEditor/HeadingLevelDropdown',
+	component: HeadingLevelDropdown,
+	parameters: {
+		docs: {
+			canvas: { sourceState: 'shown' },
+			description: {
+				component:
+					'Dropdown for selecting a heading level (1 through 6) or paragraph (0).',
+			},
+		},
+	},
+	argTypes: {
+		value: {
+			control: { type: null },
+			description: 'The chosen heading level.',
+		},
+		options: {
+			control: 'check',
+			options: [ 1, 2, 3, 4, 5, 6 ],
+			description: 'An array of supported heading levels.',
+		},
+		onChange: {
+			action: 'onChange',
+			control: { type: null },
+			description: 'Function called with the selected value changes.',
+		},
+	},
+};
+
+export default meta;
+
+export const Default = {
+	render: function Template( { onChange, ...args } ) {
+		const [ value, setValue ] = useState( args.value );
+
+		return (
+			<HeadingLevelDropdown
+				{ ...args }
+				value={ value }
+				onChange={ ( ...changeArgs ) => {
+					setValue( ...changeArgs );
+					onChange( ...changeArgs );
+				} }
+			/>
+		);
+	},
+	args: {
+		value: 2,
+	},
+};