Sync to upstream/release/607

[aatxe]

What's changed?

• Fix up the std::iterator_traits definitions for some Luau data structures.
• Replace some of the usages of std::unordered_set and std::unordered_map with Luau-provided data structures to increase performance and reduce overall number of heap allocations.
• Update some of the documentation links in comments throughout the codebase to correctly point to the moved repository.
• Expanded JSON encoder for AST to support singleton types.
• Fixed a bug in luau-analyze where exceptions in the last module being checked during multithreaded analysis would not be rethrown.

New type solver

• Introduce a refine type family to handle deferred refinements during type inference, replacing the old RefineConstraint.
• Continued work on the implementation of type states, fixing some known bugs/blockers.
• Added support for variadic functions in new non-strict mode, enabling broader support for builtins and the Roblox API.

Internal Contributors

Co-authored-by: Aaron Weiss [email protected]
Co-authored-by: Alexander McCord [email protected]
Co-authored-by: Andy Friesen [email protected]
Co-authored-by: Vyacheslav Egorov [email protected]