Fixes #3939: `\yii\Inflector::slug()` improvements:

- Added protected `\yii\Inflector::transliterate()` that could be replaced with custom translit implementation.
- Added proper tests for both intl-based slug and PHP fallback.
- Removed character maps for non-latin languages.
- Improved overall slug results.
- Added note about the fact that intl is required for non-latin languages to requirements checker.
Merge pull request #3972 from yiisoft/inflector-slug-improvements

Fixes #3939: `\yii\Inflector::slug()` improvements