Sync to upstream/release/534

[zeux]

• Implement support for __len metamethod for tables, and a rawlen builtin function (RFC RFC: Support __len metamethod for tables and rawlen function #536)
• Fix a bug in parser that erroneously permitted type packs with named elements in generic types, e.g. Foo<(a: number, b: number)> isn't valid syntax
• Fix a bug in parser that erroneously truncated hexadecimal or binary literals that are longer than 64-bit; these literals are now rejected (Luau incorrectly parses numbers #518)
• Improve performance of iteration through tables