Implement `noSquashMarkers`

[spdiswal]

Adapted from the no-squash-commits rule in v1.

Configuration

None.

Unit tests

• When there are no commits with squash markers, it does not raise any concerns.
• When there is 1 commit with a squash marker of amend!, it raises 1 concern about the subject line.
• When there is 1 commit with a squash marker of fixup!, it raises 1 concern about the subject line.
• When there is 1 commit with a squash marker of squash!, it raises 1 concern about the subject line.
• When there is 1 commit with 2 squash markers, it raises 1 concern about the subject line.
• When there are 2 commits with 1 squash marker each, it raises 2 concerns; 1 for each subject line.