refactor(ast): Introduce flexible serialization encoding for AST (#11100)

**Description:**

Following our previous discussion, I am trying to introduce a
self-describing serialization encoding for ast to replace
#11069 .

Since `serde` is currently used for json serialization, it is hard to
modify serde to support both json and binary encodings (due to the
`untagged` and `flatten` attr). So I made our own `Encode` and `Decode`
proc macro, which also facilitate unknown variant support.

<img width="1804" height="1776" alt="benchmark"
src="https://github.com/user-attachments/assets/73e294cd-be88-43a0-a390-fdd24c8f464f"
/>

I've done some simple benchmarks with a modified swc ast, and I believe
we don't have an unacceptable performance loss relative to `rkyv`.
I'm using `cbor4ii` here, which I developed, so I'd prefer it if there's
no performance difference. but it's easy to migrate to `bincode` or
other scheme.

This also introduces an `unknown` feature for `ecma_ast` crate, which
should only be enabled by the wasm plugin to ensure that old plugins can
decode new AST.

Author: quininer

.changeset/chatty-brooms-drop.md

@@ -0,0 +1,7 @@
+---
+ast_node: major
+from_variant: major
+swc_atoms: major
+---
+
+refactor(ast): Introduce flexible serialization encoding for ast