Add all the files from the interpreter repo

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

pkg/ast/ast.go

@@ -0,0 +1,39 @@
+package ast
+
+import (
+	"bytes"
+)
+
+type Node interface {
+	TokenLiteral() string
+	String() string
+}
+
+type Statement interface {
+	Node
+	statementNode()
+}
+
+type Expression interface {
+	Node
+	expressionNode()
+}
+
+type Program struct {
+	Statements []Statement
+}
+
+func (p *Program) TokenLiteral() string {
+	if len(p.Statements) > 0 {
+		return p.Statements[0].TokenLiteral()
+	}
+	return ""
+}
+
+func (p *Program) String() string {
+	var out bytes.Buffer
+	for _, stmt := range p.Statements {
+		out.WriteString(stmt.String())
+	}
+	return out.String()
+}