feat(functions): add information functions

lib/src/functions/info.dart

@@ -0,0 +1,71 @@
+import '../evaluator.dart';
+import '../values.dart';
+import 'helpers.dart';
+
+void registerInfo(Map<String, CellFunction> registry) {
+  registry['ISERROR'] = _guarded((value) => value is ErrorValue);
+  registry['ISERR'] = _guarded((value) => value is ErrorValue && value.kind != ErrorKind.na);
+  registry['ISNA'] = _guarded((value) => value is ErrorValue && value.kind == ErrorKind.na);
+  registry['ISNUMBER'] = _guarded((value) => value is NumberValue);
+  registry['ISTEXT'] = _guarded((value) => value is TextValue);
+  registry['ISLOGICAL'] = _guarded((value) => value is BooleanValue);
+  registry['ISNONTEXT'] = _guarded((value) => value is! TextValue);
+
+  registry['ISBLANK'] = (args, ctx) {
+    arity(args, 1, 1);
+    return BooleanValue(ctx.eval(args[0]) is BlankValue);
+  };
+
+  registry['NA'] = (args, ctx) {
+    arity(args, 0, 0);
+    throw const EvalException(ErrorValue(ErrorKind.na));
+  };
+
+  registry['N'] = (args, ctx) {
+    arity(args, 1, 1);
+    final value = ctx.eval(args[0]);
+    switch (value) {
+      case NumberValue(:final value):
+        return NumberValue(value);
+      case BooleanValue(:final value):
+        return NumberValue(value ? 1 : 0);
+      case ErrorValue():
+        throw EvalException(value);
+      default:
+        return const NumberValue(0);
+    }
+  };
+
+  registry['TYPE'] = (args, ctx) {
+    arity(args, 1, 1);
+    try {
+      final value = ctx.eval(args[0]);
+      return NumberValue(_typeCode(value).toDouble());
+    } on EvalException {
+      return const NumberValue(16);
+    }
+  };
+}
+
+int _typeCode(CellValue value) {
+  switch (value) {
+    case NumberValue():
+    case BlankValue():
+      return 1;
+    case TextValue():
+      return 2;
+    case BooleanValue():
+      return 4;
+    case ErrorValue():
+      return 16;
+  }
+}
+
+CellFunction _guarded(bool Function(CellValue) predicate) => (args, ctx) {
+      arity(args, 1, 1);
+      try {
+        return BooleanValue(predicate(ctx.eval(args[0])));
+      } on EvalException catch (error) {
+        return BooleanValue(predicate(error.error));
+      }
+    };