Update exercise difficulties

[glennj]

• Change the ordering in the "sorted" workflow: difficulty, then name
• The add-practice-exercise script keeps config.json in sorted order

New ordering

+----+-----------------------+------+
|  n | exercise              | diff |
+----+-----------------------+------+
|  1 | hello-world           |    1 |
|  2 | difference-of-squares |    2 |
|  3 | leap                  |    2 |
|  4 | raindrops             |    2 |
|  5 | resistor-color        |    2 |
|  6 | resistor-color-duo    |    2 |
|  7 | rna-transcription     |    2 |
|  8 | strain                |    2 |
|  9 | two-fer               |    2 |
| 10 | acronym               |    3 |
| 11 | armstrong-numbers     |    3 |
| 12 | bob                   |    3 |
| 13 | eliuds-eggs           |    3 |
| 14 | isogram               |    3 |
| 15 | pangram               |    3 |
| 16 | resistor-color-trio   |    3 |
| 17 | rotational-cipher     |    3 |
| 18 | space-age             |    3 |
| 19 | all-your-base         |    4 |
| 20 | allergies             |    4 |
| 21 | anagram               |    4 |
| 22 | atbash-cipher         |    4 |
| 23 | binary-search         |    4 |
| 24 | matching-brackets     |    4 |
| 25 | phone-number          |    4 |
| 26 | protein-translation   |    4 |
| 27 | roman-numerals        |    4 |
| 28 | word-count            |    4 |
| 29 | binary-search-tree    |    5 |
| 30 | change                |    5 |
| 31 | flower-field          |    5 |
| 32 | run-length-encoding   |    5 |
| 33 | prism                 |    6 |
+----+-----------------------+------+

[glennj]

@BNAndras one thing I didn't notice is the bin/sort-practice-exercises script. Given the change to the add script, do you think the sorter is still needed?

[BNAndras]

Eh, the sorter is probably not needed with this track's tooling setup.