feat(full-text-search): add an optional score explanation

[Viatorus]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/LokiJS-Forge/LokiJS2/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[coveralls]

Coverage decreased (-0.006%) to 92.407% when pulling f005c50 on fts_explain into 9b926e2 on master.

[coveralls]

Coverage decreased (-0.006%) to 92.407% when pulling f005c50 on fts_explain into 9b926e2 on master.