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

do not use scripting as it is disabled by default and will cause edge
case queries to fail

fixes #4187

Fixes #4071: `mail` component renamed to `mailer`, `yii\log\EmailTarget::$mail` renamed to `yii\log\EmailTarget::$mailer`

- Removed `form_begin`, `form_end`
- Added `use()` and `ViewRenderer::uses` that are importing classes and namespaces
- Added widget dynamic functions `*_begin`, `*_end`, `*_widget`, `widget_end`
- Added more tests

fixes #3587

issue #3520

breaking changes.