refactor(ast): reduce #[cfg_attr] boilerplate in AST type defs (#4375)

Remove a ton of `#[cfg_attr(feature = "serialize", serde(...))]` boilerplate from AST type definitions.

Before: `#[cfg_attr(feature = "serialize", serde(flatten))]`
After: `#[serde(flatten)]`

This is a reprise of #2669, which was later reverted, but this time doing it using our existing zero-cost `#[ast]` dummy macro attr, so no compile time penalty this time around.

This makes no difference to either runtime or compile time behavior, purely removes the `cfg_attr` boilerplate and makes the code easier to read.