Refactor module slug references with constants

[johnPhillips]

Feature Description

Following on from a comment on a recent pull request, it might be a good idea to establish module slugs as module-level constants rather than typing the string whenever required.

/**
 * assets/js/modules/analytics/constants.js 
 */
export const MODULE_SLUG = 'analytics';

// An additional alias might also be useful when multiple slugs are referenced in a file
export { MODULE_SLUG as MODULE_SLUG_ANALYTICS };

/**
 * Then, elsewhere:
 */
import { MODULE_SLUG } from '../../constants'; 
import { MODULE_SLUG_ANALYTICS } from 'assets/js/modules/analytics/constants';

const ideaHubConnected = useSelect( ( select ) => select( CORE_MODULES ).isModuleConnected( MODULE_SLUG ) );
const analyticsConnected = useSelect( ( select ) => select( CORE_MODULES ).isModuleConnected( MODULE_SLUG_ANALYTICS ) );

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