support for Decimal.isInfitite and isNan

lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjDecimalSupport.kt

@@ -94,6 +94,12 @@ object ObjDecimalSupport {
         decimalClass.addFn("toReal") {
             ObjReal.of(valueOf(thisObj).doubleValue(false))
         }
+        decimalClass.addFn("isInfinite") {
+            ObjFalse
+        }
+        decimalClass.addFn("isNaN") {
+            ObjFalse
+        }
         decimalClass.addFn("toString") {
             ObjString(valueOf(thisObj).toStringExpanded())
         }

lynglib/src/commonMain/kotlin/net/sergeych/lyng/stdlib_included/decimal_lyng.kt

@@ -161,6 +161,10 @@ extern class Decimal() {
     extern fun toInt(): Int
     /** Convert to `Real`. */
     extern fun toReal(): Real
+    /** Return true if this decimal is positive or negative infinity. Always false for Decimal. */
+    extern fun isInfinite(): Bool
+    /** Return true if this decimal is NaN (not a number). Always false for Decimal. */
+    extern fun isNaN(): Bool
     /**
      * Convert to a plain decimal string without scientific notation.
      *