Fix Function.toString (#1411)

Fixes an issue where the underlying default Java toString() is leaking
through

pkl-core/src/test/files/LanguageSnippetTests/input/basic/string.pkl

@@ -43,6 +43,10 @@ examples {
     "\(new Person2 { name = "Pigeon"; age = 42 })"
     "\(null)"
     "\(Null(new Person { name = "Pigeon"; age = 42 }))"
+    "\(() -> null)"
+    "\((p1) -> null)"
+    "\((p1, p2) -> null)"
+    "\((p1, p2, p3) -> null)"
   }
 
   ["escaping"] {

pkl-core/src/test/files/LanguageSnippetTests/input/lambdas/lambda1.pkl

@@ -9,4 +9,7 @@ res3 = even.apply(11)
 
 local mult = (x, y) -> x * y
 
-res4 = mult.apply(2, 3)
+res4 = mult.apply(2, 3)
+
+res5 = even.toString()
+res6 = mult.toString()

pkl-core/src/test/files/LanguageSnippetTests/output/basic/string.pcf

@@ -34,6 +34,10 @@ examples {
     "My name is Pigeon and I'm 42 years old."
     "null"
     "null"
+    "new Function0 {}"
+    "new Function1 {}"
+    "new Function2 {}"
+    "new Function3 {}"
   }
   ["escaping"] {
     "\\\"\\("

pkl-core/src/test/files/LanguageSnippetTests/output/lambdas/lambda1.pcf

@@ -2,3 +2,5 @@ res1 = "abc"
 res2 = true
 res3 = false
 res4 = 6
+res5 = "new Function1 {}"
+res6 = "new Function2 {}"