[ci skip]

Improved checks where substr_compare is used, replaced more substr + comapre cases with substr_compare

`yii\mongodb\Collection::buildRegexCondition()` added.

fixes #4086

fixes #4012

[ci skip]

```
vendor/bin/indent --spaces -r framework/
vendor/bin/indent --tabs -r . --pattern=*.json
vendor/bin/indent --spaces -r . --pattern=*.md
vendor/bin/indent --spaces -r . --pattern=yii.*.js
vendor/bin/indent --spaces -r . --pattern=*.css
```

Fixes #2955: Changed the signature of ActiveQuery constructors and `ActiveRecord::createQuery()` to simplify customizing ActiveQuery classes

fixes #1946

issue #2892

so information about changed values is available in `afterSave`-event.

fixes #2892