Add `vanilla` flag for generators

[cowtowncoder]

With 3.0 we can start specializing generators (and parsers too) more, due to full immutability of feature flags.

One thing that should allow minor performance optimization is separating "non-vanilla" feature handling, such as:

• Unquoted field names
• Pretty-printing
• Custom escaping

into either fully separate generator implementations, or, as the first step, to use "vanilla flag" (similar to how BeanDeserializer works) to at least combine many checks into one, streamlining the default case of not requiring additional checks.