Implement rules for preferences for true/false/null

[osamayelk]

Description

Added new rules for prefer toBeNull/toBeTrue/toBeFalse. This implements the enhancement #352
We could discuss whether or not we need rules to prefer toBeTrue/toBeFalse over toBeTruthy/toBeFalsy and what should we call them.

How has this been tested?

I wrote mocha tests following the same pattern for toBeUndefined

Types of changes

• Test change (non-breaking change which adds additional test scenarios)
• Refactor change (non-breaking change updates coding styles)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have ran npm run test and everything passes
• My code follows the code style of this project
• I have updated the documentation where necessary
• I have read the CONTRIBUTING document
• I have added tests to cover my changes
• All new and existing tests pass

[pullard]

Sounds like a great addition, is this being looked at?