refactor: remove plugins code

plugins/README.md

-Goals: Language as Plugins
-
-Stage 1: C++ API for Golang
-
-Stage 2: Rust API for Golang
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plugins/pgroovy/bridge.go.future

-package main
-
-// #cgo CFLAGS: -DPNG_DEBUG=1
-// #include "wrap.h"
-import "C"
-
-func main() {
-	C.output()
-}
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plugins/pgroovy/wrap.cpp

-#include <iostream>
-#include "wrap.h"
-
-using namespace std;
-
-int output()
-{
-    cout << "Hello, World!";
-}
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plugins/pgroovy/wrap.h

-#ifndef WRAP_CPP_H
-#define WRAP_CPP_H
-
-#ifdef __cplusplus
-extern "C" {
-#endif // __cplusplus
-int output();
-#ifdef __cplusplus
-}
-#endif // __cplusplus
-
-#endif // WRAP_CPP_H
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