feat(tvix/eval): make into_json public

Allow other crates (like tvix-glue) to look at a Value in JSON, which is
used by the structured attrs feature.

Change-Id: Iba02ace6e11a74c3f9b19dcbef4b008b76dec046
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10602
Reviewed-by: tazjin <tazjin@tvl.su>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/eval/src/value/json.rs

@@ -12,7 +12,7 @@ use serde_json::Value as Json; // name clash with *our* `Value`
 use serde_json::{Map, Number};
 
 impl Value {
-    pub(crate) async fn into_json(
+    pub async fn into_json(
         self,
         co: &GenCo,
     ) -> Result<Result<Json, CatchableErrorKind>, ErrorKind> {