Added `searchPlaceholder` prop to config, as an alternative to `searchPlaceHolder`

[talisraeli]

• Fixes #355
• Added searchPlaceholder prop to config.ts.
• Added DEFAULT_SEARCH_PLACEHOLDER to config.ts - defines the default search placeholder string (defaulted to Search).
• Maps old searchPlaceHolder and new searchPlaceholder to the same prop when using useSearchPlaceHolderConfig().
• ✅ Successfully passed testing on Storybook:
  • ✅ With custom searchPlaceHolder.
  • ✅ With custom searchPlaceholder.
  • ✅ Without custom searchPlaceHolder or searchPlaceholder.