feat(functions): add builtin function registry

lib/src/functions.dart

@@ -0,0 +1,26 @@
+import 'evaluator.dart';
+import 'functions/aggregate.dart';
+import 'functions/date.dart';
+import 'functions/financial.dart';
+import 'functions/info.dart';
+import 'functions/logical.dart';
+import 'functions/lookup.dart';
+import 'functions/math.dart';
+import 'functions/stats.dart';
+import 'functions/text.dart';
+
+Map<String, CellFunction> buildFunctions() {
+  final registry = <String, CellFunction>{};
+  registerMath(registry);
+  registerAggregate(registry);
+  registerStats(registry);
+  registerText(registry);
+  registerLogical(registry);
+  registerInfo(registry);
+  registerLookup(registry);
+  registerDate(registry);
+  registerFinancial(registry);
+  return registry;
+}
+
+List<String> functionNames() => (buildFunctions().keys.toList())..sort();