[META] Planned changes to AST structure

Discussing with @mlechu, I think there's a few near term changes we'd like to make to the core JuliaSyntax AST data structures. I'm opening up this issue so we have a place to track them all and discuss the overall plan. Particulars of any change can go to the appropriate issue or PR. I think the plan would be that we put them all in and then tag JuliaSyntax 2.0 for 1.13 to go with putting JuliaLowering in Base.

Planned changes (will amend list as more things come up).

1. [ ] #564
2. [x] Remove the DOTOP_FLAG - it's hardly used at this point anyway (#568)
3. [x] Find a different representation for the numeric bits - they are only used by two heads, but take up 8 bits in every syntax node. This seems a bit wasteful.
4. [ ] Remove individual syntax heads for unicode operators - replace with one head per precedence level (plus possibly a few special heads used elsewhere)