docs: skip autolink for hidden Native fns in the changelog

`mix publisho` promoted the RELEASE.md notes into CHANGELOG.md, an
ex_doc extra. Those notes reference the hidden
`Emily.Native.async_eval/2` and `Emily.Native.fast_rope_int/8` NIF
stubs, so `mix docs --warnings-as-errors` (the precommit / CI docs gate)
failed on the unresolved autolinks. Add both to `:skip_code_autolink_to`
and note the fix in RELEASE.md.

Author: ausimian

RELEASE.md

@@ -0,0 +1,5 @@
+### Fixed
+
+- Documentation no longer fails to build over autolink references to the
+  hidden `Emily.Native.async_eval/2` and `Emily.Native.fast_rope_int/8`
+  NIF stubs in the changelog; both are excluded from ex_doc autolinking.

mix.exs

@@ -222,6 +222,8 @@ defmodule Emily.MixProject do
         "Emily.Native.from_binary/3",
         "Emily.Native.conv_general/8",
         "Emily.Native.worker_queue_depth/1",
+        "Emily.Native.async_eval/2",
+        "Emily.Native.fast_rope_int/8",
         # Native Expr-compiler internals are `@moduledoc false`.
         "Emily.IR",
         "Emily.Program",