Move AST nodes to their own files.

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

pkg/ast/prefix_expression.go

@@ -0,0 +1,17 @@
+package ast
+
+import "code.jmug.me/jmug/interpreter-in-go/pkg/token"
+
+type PrefixExpression struct {
+	Token    token.Token // The operator token
+	Operator string
+	Right    Expression
+}
+
+func (pe *PrefixExpression) expressionNode() {}
+func (pe *PrefixExpression) TokenLiteral() string {
+	return pe.Token.Literal
+}
+func (pe *PrefixExpression) String() string {
+	return "(" + pe.Operator + pe.Right.String() + ")"
+}