Implement presence checks

New functions:
- `envp` checks if env is defined
- `filep` checks if file exists

functions.go

@@ -7,7 +7,9 @@ var funcMap map[string]any
 func init() {
 	funcMap = make(map[string]any)
 	funcMap["file"] = File
+	funcMap["filep"] = Filep
 	funcMap["env"] = Env
+	funcMap["envp"] = Envp
 }
 
 // File reads a file and returns its content as a string
@@ -19,7 +21,19 @@ func File(name string) string {
 	return string(data)
 }
 
+// Filep checks if a file exists
+func Filep(name string) bool {
+	_, err := os.Stat(name)
+	return err == nil
+}
+
 // Env expands environment variables in a string
 func Env(text string) string {
 	return os.ExpandEnv(text)
 }
+
+// Envp checks if an environment variable is set
+func Envp(name string) bool {
+	_, ok := os.LookupEnv(name)
+	return ok
+}