chore: Make std.setDiff accepts str. (#478)

Motivation:

Make std.setDiff accepts str

refs: #477

Author: He-Pin

sjsonnet/src/sjsonnet/Std.scala

@@ -1744,8 +1744,8 @@ class Std(
         validateSet(ev, pos, keyF, args(0))
         validateSet(ev, pos, keyF, args(1))
 
-        val a = toSetArr(args, 0, pos, ev)
-        val b = toSetArr(args, 1, pos, ev)
+        val a = toArrOrString(args(0), pos, ev)
+        val b = toArrOrString(args(1), pos, ev)
         val out = new mutable.ArrayBuffer[Lazy]
 
         for (v <- a) {

sjsonnet/test/src/sjsonnet/StdSetDiffTests.scala

@@ -0,0 +1,18 @@
+package sjsonnet
+
+import sjsonnet.TestUtils.eval
+import utest.*
+
+object StdSetDiffTests extends TestSuite {
+
+  def tests: Tests = Tests {
+    test {
+      eval("""
+          |{
+          |  local options = ['a', 'b', 'c'],
+          |  diff: std.setDiff('d', options),
+          |}
+          |""".stripMargin).toString ==> """{"diff":["d"]}"""
+    }
+  }
+}