Adding stylish formatter to replace pretty formatter (deprecated)

[scalvert]

We've spent a lot of time with the Pretty formatter, in multiple incarnations. The latest, which leverages the excellent Ink library, provides some pretty robust functionality, but sadly has also resulted in a lot of maintenance burden. The amount of benefit it gives is outweighed by the additional maintenance cost, all for a formatter.

As such, we're deprecating the pretty formatter and its associated infra (@checkup/ui package). This will be done in a follow up PR.

To replace it, we're outputting the SARIF in a familiar format used by other static analysis tools, such as eslint and ember-template-lint.

New stylish formatter:

This formatter has a number of improvements over the pretty formatter:

1. It works with SARIF, and is very fault-tolerant
2. It outputs both location information, and category
3. It provides a useful summary
4. It resembles other static analysis tool outputs.