[WebAssembly] Add -wasm-enable-exnref option (#93597)

This adds `-wasm-enable-exnref`, which will enable the new EH
instructions using `exnref` (adopted in Oct 2023 CG meeting):
https://github.com/WebAssembly/exception-handling/blob/main/proposals/exception-handling/Exceptions.md
This option should be used with `-wasm-enable-eh`.