New "Count" Report Format

[jack-worman]

This is a format that displays the count of each issue type.
I requested this feature in #8225.

I look forward to being able to run: psalm --ignore-baseline --output-format=count

For example, the output of the above command on this code base is:

PossiblyUndefinedStringArrayOffset: 1
PossiblyUnusedProperty: 1
RedundantCondition: 1
ReferenceConstraintViolation: 1
UnusedClass: 1
UnusedPsalmSuppress: 1
InvalidPropertyAssignmentValue: 2
DocblockTypeContradiction: 3
PropertyNotSetInConstructor: 5
DeprecatedClass: 35
DeprecatedMethod: 44
DeprecatedProperty: 89
PossiblyUndefinedIntArrayOffset: 144

[AndrolGenhald]

LGTM. The only thing possibly worth discussing is the sorting, but sorting by issue count seems fine.

[orklah]

Thanks!