Implement `noSingleWordSubjectLines`

[spdiswal]

Adapted from the multi-word-subject-lines rule in v1.

Configuration

None.

Unit tests

• When all commits have subject lines of at least two words, it does not raise any concerns.
• When there is 1 commit with a subject line of only one word, it raises 1 concern about the subject line.
• When there are 2 commits with subject lines of only one word, it raises 2 concerns; 1 for each subject line.