Raise PHPStan to the max level

[chrisjenkinson]

This raises PHPStan from level 5 to max for src/ and tests/ and fixes the 25 errors that surfaced:

• Arrays have element types in docblocks, for example RegexFinder::getMatches(), MatchedText::getAll(), NodeTraverser::traverseChildren() and the AmbiguousTokenFoundException getters.
• TokenStream::expectTokenType() and expectTokenTypes() check the current token for null directly instead of relying on a separate count.
• TokenInterface gains getText(): string for the text a token consumed. Token checks when it's constructed that its all value is a string, so the lexer and __toString() can rely on it without a check of their own. getValue('all') still works.
• NodeTraverserTest asserts that traverse() returned a node before using it, since it now returns ?NodeInterface.

spec/ is still excluded. PHPStan can't follow phpspec's magic methods and reports around 300 errors there, and the only phpspec extension for PHPStan targets PHPStan 0.11 and phpspec 6.

Breaking changes: implementations of TokenInterface need getText(), and Token throws InvalidArgumentException if its all value is missing or isn't a string.