Move AST nodes to their own files.

Signed-off-by: jmug <u.g.a.mariano@gmail.com>

pkg/ast/expression_statement.go

@@ -0,0 +1,23 @@
+package ast
+
+import "code.jmug.me/jmug/interpreter-in-go/pkg/token"
+
+// ExpressionStatement is a simple wrapper of an expression in a statement
+// This is common in scripting languages and allows you to have a source line
+// that is solely an expression, think of the Python REPL and how you can
+// type `1 + 1` and get a result.
+type ExpressionStatement struct {
+	Token      token.Token // The first token in the expression.
+	Expression Expression
+}
+
+func (es *ExpressionStatement) statementNode() {}
+func (es *ExpressionStatement) TokenLiteral() string {
+	return es.Token.Literal
+}
+func (es *ExpressionStatement) String() string {
+	if es.Expression != nil {
+		return es.Expression.String()
+	}
+	return ""
+}