Apply #[ast] macro to AST types in oxc_syntax crate

[overlookmotel]

#4614 made all AST types in oxc_ast crate #[repr(C)] via the #[ast] macro.

We still need to apply #[ast] macro to types in oxc_syntax crate which are part of AST e.g. AssignmentOperator.

We could also put #[ast] on oxc_span::Span. Or we may just want to put #[repr(C)] on it, and hard-code the layout into AST codegen, since Span is such a simple type.

I don't think there are any other AST types in other crates - apart from Atom, Vec and Box, which we treat as special cases.