context.go splitted in two files: expr-context.go and expr-function-go.

Expr interface moved from ast.go to the new file expr.go

expr.go

@@ -0,0 +1,11 @@
+// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// All rights reserved.
+
+// expr.go
+package expr
+
+// ----Expression interface
+type Expr interface {
+	Eval(ctx ExprContext) (result any, err error)
+	String() string
+}