refactor: replace deprecated String.prototype.substr()

.substr() is deprecated so we replace it with .slice() which works similarily but isn't deprecated

Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

lib/index.js

@@ -59,8 +59,8 @@ function toIdentifierCase( text ) {
 				return word.toLowerCase();
 			}
 			return (
-				word.substr( 0, 1 ).toUpperCase()
+				word.slice( 0, 1 ).toUpperCase()
-				+ word.substr( 1 ).toLowerCase()
+				+ word.slice( 1 ).toLowerCase()
 			);
 		} )
 		.join( '' );