5.0.0: Integrate parser

• Merged PR #8: Add support for match expression - @thekid
• Added builtin support for null-safe instance operator ?->, see
  https://wiki.php.net/rfc/nullsafe_operator & xp-framework/compiler#9
  (@thekid)
• Merged PR #7: PHP8 attributes support - @thekid
• Integrated throw expressions, see xp-framework/compiler#85 - @thekid
• Fixed missing support for insteadof trait keyword - @thekid
• Merged PR #1: Integrate parser
  • Migrate tokenizer, parser and language from xp-framework/compiler
  • Added enclosing type to constants, properties and methods
  • Added lang.ast.Node::is() method to check for node kind
  • Added lang.ast.nodes.TypeDeclaration::declare() in favor of inject.
  • Added new parse tree API
  • Added new visitor API
  • Refactored types API to dedicated package lang.ast.types
  • Added dedicated classes for nullable, literal and value types
    (@thekid)