Convert multiple exercises to named exports

[tejasbubane]

Instead of default exports

Issue: #436

These are all the trivial changes - hence all at once. After this we have only 7 left which are not so trivial - they will be separate PRs - that would be easier to review.

➜  ag "export default"
exercises/variable-length-quantity/example.js
27:export default {

exercises/strain/example.js
18:export default Strain;

exercises/palindrome-products/example.js
38:export default Palindromes;

exercises/queen-attack/example.js
52:export default QueenAttack;

exercises/sum-of-multiples/example.js
19:export default Sum;

exercises/sieve/example.js
30:export default function (n) {

exercises/hexadecimal/example.js
1:export default function (hex) {

[SleeplessByte]

You're the best @tejasbubane !!! 🥇