Implement strings and partial implementation of arrays (missing index expr eval)

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

pkg/ast/index.go

@@ -0,0 +1,21 @@
+package ast
+
+import (
+	"fmt"
+
+	"code.jmug.me/jmug/interpreter-in-go/pkg/token"
+)
+
+type IndexExpression struct {
+	Token token.Token // The "[" token
+	Left  Expression
+	Index Expression
+}
+
+func (ie *IndexExpression) expressionNode() {}
+func (ie *IndexExpression) TokenLiteral() string {
+	return ie.Token.Literal
+}
+func (ie *IndexExpression) String() string {
+	return fmt.Sprintf("(%s[%s])", ie.Left.String(), ie.Index.String())
+}