Complete the specification for formatting

I started writing a doc there: https://github.com/GDQuest/GDScript-formatter/blob/main/docs/specification_formatter.md

Things left to cover:

- Nested constructs (like nested function calls)
- Comments in constructs (like comments in the middle of function calls, inline after commas, above arguments, after last argument without trailing comma...)
- Expressions:
- ternaries, whether to keep an entire per line, where to wrap (after if, before else, etc.)
- Boolean expressions, mainly replicating the official styleguide + deciding on horizontal packing (one expression segment per line, balanced across the minimal number of lines possible, greedy packed? + continuation layout)
- Chains of function calls
- Additions, multiplications, etc. (binary operators)