Make 'default' param of json.decode keyword or positional

Following the discussion at https://github.com/google/starlark-go/pull/466

RELNOTES: the 'default' param of json.decode can now be used as a keyword
parameter.
PiperOrigin-RevId: 535643654
Change-Id: Ie203548ba46e037db6ce2303bfcd156edf7e96c9

src/main/java/net/starlark/java/lib/json/Json.java

diff --git a/src/main/java/net/starlark/java/lib/json/Json.java b/src/main/java/net/starlark/java/lib/json/Json.java
index c13da7e..608a046 100644
--- a/src/main/java/net/starlark/java/lib/json/Json.java
+++ b/src/main/java/net/starlark/java/lib/json/Json.java
@@ -290,17 +290,16 @@
               + " string occurs more than once in the object, the last value for the key is kept.\n"
               + "<li>a JSON array is parsed as new unfrozen Starlark list.\n"
               + "</ul>\n"
-              + "If <code>x</code> is not a valid JSON encoding and the optional, positional-only"
+              + "If <code>x</code> is not a valid JSON encoding and the optional"
               + " <code>default</code> parameter is specified (including specified as"
               + " <code>None</code>), this function returns the <code>default</code> value.\n"
               + "If <code>x</code> is not a valid JSON encoding and the optional"
-              + " <code>default</code> parameter is <em>not</em> specified, this"
-              + " function fails.",
+              + " <code>default</code> parameter is <em>not</em> specified, this function fails.",
       parameters = {
         @Param(name = "x", doc = "JSON string to decode."),
         @Param(
-            // positional-only to match Starlark language spec for dict.get(), dict.pop(), getattr()
             name = "default",
+            named = true,
             doc = "If specified, the value to return when <code>x</code> cannot be decoded.",
             defaultValue = "unbound")
       },

src/test/java/net/starlark/java/eval/testdata/json.star

diff --git a/src/test/java/net/starlark/java/eval/testdata/json.star b/src/test/java/net/starlark/java/eval/testdata/json.star
index bf81227..0ce226c 100644
--- a/src/test/java/net/starlark/java/eval/testdata/json.star
+++ b/src/test/java/net/starlark/java/eval/testdata/json.star
@@ -151,15 +151,13 @@
 
 ## json.decode with default specified
 
+assert_eq(json.decode('{"valid": "json"}', default = "default value"), {"valid": "json"})
 assert_eq(json.decode('{"valid": "json"}', "default value"), {"valid": "json"})
+assert_eq(json.decode('{"invalid": "json"', default = "default value"), "default value")
 assert_eq(json.decode('{"invalid": "json"', "default value"), "default value")
+assert_eq(json.decode('{"invalid": "json"', default = None), None)
 assert_eq(json.decode('{"invalid": "json"', None), None)
 
-assert_fails(
-    lambda: json.decode('{"invalid": "json"', default = "default value"),
-    "got named argument for positional-only parameter 'default'",
-)
-
 def codec(x):
     return json.decode(json.encode(x))