Add all the files from the interpreter repo

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

pkg/ast/identifier.go

@@ -0,0 +1,20 @@
+package ast
+
+import "code.jmug.me/jmug/interpreter-in-go/pkg/token"
+
+// Identifier is treated as an expression because in certain
+// circumstances they can return values (think `let some = other` where `other`
+// is actually an expression returning a value) and this makes them easier to
+// handle (according to the author).
+type Identifier struct {
+	Token token.Token
+	Value string
+}
+
+func (i *Identifier) expressionNode() {}
+func (i *Identifier) TokenLiteral() string {
+	return i.Token.Literal
+}
+func (i *Identifier) String() string {
+	return i.Value
+}