starlark/lib/json: a JSON module for Starlark

This CL ports the go.starlark.net/starlarkjson module from Go to Java.
The json module provides four functions:

  json.decode
  json.encode
  json.indent        (not yet implemented)
  json.encode_indent (not yet implemented)

It is tested through eval.ScriptTest, which adds the json module to
its environment, along with 'struct', a simple struct-like type.

Some tests are commented out, awaiting StarlarkFloat, or richer string support.

This is a first step towards removing Bazel's struct.to_json.

Updates bazelbuild/starlark#83

PiperOrigin-RevId: 337944489

src/BUILD

@@ -449,6 +449,7 @@ filegroup(
         "//src/main/java/com/google/devtools/common/options:srcs",
         "//src/main/java/net/starlark/java/cmd:srcs",
         "//src/main/java/net/starlark/java/spelling:srcs",
+        "//src/main/java/net/starlark/java/lib/json:srcs",
         "//src/main/native:srcs",
         "//src/main/protobuf:srcs",
         "//src/main/tools:srcs",

src/main/java/net/starlark/java/lib/json/BUILD

@@ -0,0 +1,21 @@
+load("@rules_java//java:defs.bzl", "java_library")
+
+licenses(["notice"])
+
+filegroup(
+    name = "srcs",
+    srcs = glob(["**"]),
+    visibility = ["//src:__subpackages__"],
+)
+
+# Starlark json module
+java_library(
+    name = "json",
+    srcs = ["Json.java"],
+    visibility = ["//src/main/java/net/starlark/java:clients"],
+    deps = [
+        "//src/main/java/net/starlark/java/annot",
+        "//src/main/java/net/starlark/java/eval",
+        "//src/main/java/net/starlark/java/syntax",
+    ],
+)