Restore ability to generate documentation JSON blob.

[dmsnell]

This patch makes a few updates to unblock generating the documentation for WordPress, notably updating the underlying dependency nikic/php-parser from ^1.0 to ^3.0. Some updating is required here while some is required on the intermediate dependency on phpDocumentor/Reflection.

• Depend on the fork dmsnell/Reflector which updates the PHP parser.
• Changes from the PHPParser namespace/class aliases with underscores to the normal form. For example, from \PHPParser_Node to \PHPParser\Node.
• Updates to AST changes in PHPParser: notably how names are represented and anonymous objects.

Because nikic/php-parser made a change in how it presents names, prefixing every global identifier with the namespace prefix, a cleanup pass has been added to remove it. This is optional and could be removed if the documentation wants to adopt the global namespace prefix (and there are reasons to consider that).

Changes to the phpDocumentor/Reflection dependency involve:

• Conditionally relying on \Stringable for runs with PHP versions lacking that interface.
• Update deprecations in certain type definitions.
• Update naming
• Support nullable types in the AST.

Overall there isn’t much work, and this will parse trunk with PHP 7.4. One patch was necessary in Core to unblock some syntax issues, though the syntax form would have been solvable some other way — it was easier to remove it than update the parser.

See also #251

[dmsnell]

Rebuilt series of commits after previous head of adbc85c