Add PrettyConfig::braced_structs

[juntyr]

This is just a quick attempt at getting something going for #550. Anyone (e.g. @HKalbasi) is free to pick up my branch and work on this further, I might not have time for more for a while.

• Add tests, especially for the newtype interaction
• Add custom error variants / messages
• Update the extension docs and grammar
• I've included my change in CHANGELOG.md

[juntyr]

Thinking about this some more and the challenges of this implementation draft ... I think this should not be an extension but instead a pretty printing option

• this would allow all parsers to parse both the parenthesised and the braced syntax
• it would simplify the parser because it would not need to suddenly make previously allowed cases fail
• it would not require changes to RawValue
• as a downside, it would not be possible (immediately) to only parse code that follows the new syntax