PHP 7.4 update

Included in this release:

• Increased minimum requirement to PHP 7.4.
• Added the possibility to register custom tag types.
• Added setSeparatorChar() to change the separator character.
• Added setUppercase() to convert version tags to uppercase.
• Added support for the snapshot version tag type.
• Added isSnapshot().
• Using strict types in all classes.
• Set up PHPStan analysis, clean up to level 9.