Tighten and modernise the public API

[chrisjenkinson]

This tightens the public API and moves the code to modern PHP property syntax:

• StateInterface, NodeInterface, NodeVisitorInterface and TokenInterface declare native return types. findMatchingToken() returns TokenInterface instead of TokenInterface|false, and leaveNode() returns NodeInterface|NodeVisitorAction|null.
• VisitableInterface::accept() takes a VisitorInterface and returns mixed. The visitor interfaces have docblocks explaining how to extend them. Token::getValue() throws NonexistentKeyException for a missing key.
• StateInterface drops runMatchers(), and AbstractState's matching internals are private. Matches are held as small internal MatcherMatch objects instead of three parallel arrays.
• Lexer::pushState() replaces setState(), which remains as a deprecated alias.
• Every property is typed, with readonly and constructor promotion where they fit.

Breaking changes: implementations of these interfaces need the new return types, including leaveNode() in visitors and accept() in visitable nodes. Subclasses of AbstractState can no longer call runMatchers() or guardAgainstWrongNumberOfMatches(). Subclasses of AbstractNode that redeclare $attributes or $nodes must type them as array. Token and MatchedText now require arrays for their values.