feat(formula): add expression ast nodes

lib/src/ast.dart

@@ -0,0 +1,63 @@
+import 'address.dart';
+import 'values.dart';
+
+sealed class Expr {
+  const Expr();
+}
+
+class NumberLit extends Expr {
+  final double value;
+  const NumberLit(this.value);
+}
+
+class TextLit extends Expr {
+  final String value;
+  const TextLit(this.value);
+}
+
+class BoolLit extends Expr {
+  final bool value;
+  const BoolLit(this.value);
+}
+
+class ErrorLit extends Expr {
+  final ErrorKind kind;
+  const ErrorLit(this.kind);
+}
+
+class RefNode extends Expr {
+  final CellAddress address;
+  final String? sheet;
+  const RefNode(this.address, [this.sheet]);
+}
+
+class RangeNode extends Expr {
+  final CellAddress start;
+  final CellAddress end;
+  final String? sheet;
+  const RangeNode(this.start, this.end, [this.sheet]);
+}
+
+class NameNode extends Expr {
+  final String name;
+  const NameNode(this.name);
+}
+
+class UnaryOp extends Expr {
+  final String op;
+  final Expr operand;
+  const UnaryOp(this.op, this.operand);
+}
+
+class BinaryOp extends Expr {
+  final String op;
+  final Expr left;
+  final Expr right;
+  const BinaryOp(this.op, this.left, this.right);
+}
+
+class FuncCall extends Expr {
+  final String name;
+  final List<Expr> args;
+  const FuncCall(this.name, this.args);
+}