Refactor(cli): Rename `--default` to `--mode`
Refactor(cli): Rename `--default` to `--mode`

Reasons:
- `-m, --mode <mode>` is more descriptive than `-d, --default <mode>`,
  and fits better with `-o, --verbosity <verbosity>`.
- We call it `Mode` and `parseMode` internally.

exercism/configlet