Add `UnknownBytes` to AST nodes to make Wasm plugin backward compatible (in the correct way)

[kdy1]

#10842 is a hack, and #10841 will act as a guard for the hack.

Regardless, I'm going to add UnknownBytes to all extensible AST node type definitions. In that way, the Wasm plugin authors can return the unknown bytes as-is.

With #10842, unknown bytes will be dropped silently because those bytes are treated as a padding in some ArchivedXXX type.