Explore static rule/type information

[msujew]

A lot of low level AST/CST utilities (and even more adopter facing services such as the formatter) require usage of strings to identify attributes an AST/CST element. In an effort to make this more resilient and more easily refactorable, a few ideas have been proposed.

• Export property types for checking at compile time #1184
• Export keywords for checking at compile time #1195
• provide optional key with reflection #1206
• Constants for rule names #1207