Osama

Add option to separate pie chart slices using borders (partially implement #1439 )

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**What kind of change does this PR introduce?** (check one with "x") - [ ] Bugfix - [x] Feature - [ ] Code style update (formatting, local variables) - [...

Implement rules for preferences for true/false/null

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## 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...