Review PHPDoc classes

- Move DocBlock-related regular expressions from `Regex` to
  `PHPDocRegex`
- Move `normaliseType()` from `AbstractTag` to `PHPDoc`
- Move tag classes to `PHPDoc\Tag`, then rename and make them immutable
- Detect parameters passed by reference
- Improve parsing, e.g. when invalid whitespace is added between tokens
- Fail with an exception when a DocBlock cannot be parsed
- Remove `$legacyNullable` support
- Fix issue where `PHPDoc::$Summary` may not be `null` when a DocBlock
  has no summary
- Fix issue where `/***/` is not recognised as an invalid DocBlock

Author: lkrms