[ISSUE #789] Fix java syntax highlighting

[mewsf]

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What is the purpose of the change

Fix #789

Brief changelog

https://docusaurus.io/docs/2.x/markdown-features/code-blocks#supported-languages has a way to extend language support.

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[mewsf]

依照docusaurus官网的文档，在docusaurus.config.js修改additionalLanguages就能为代码高亮增加更多语言支持, PrimJs也确实支持Java，但是不知什么原因导致一些语言在docusaurus仍然无法支持，推测是这个版本导入依赖时有问题，有些语言的格式化代码没有引入，但是更新docusaurus不太现实。文档中给出了一种手动增加受支持语言的办法，可以通过npm run swizzle @docusaurus/theme-classic prism-include-languages将导入其他语言的功能模块文件手动导出再进行修改。这个文件本身是根据additionalLanguages设置再导入其他语言的格式，内容如下

import siteConfig from '@generated/docusaurus.config';
export default function prismIncludeLanguages(PrismObject) {
  const {
    themeConfig: {prism},
  } = siteConfig;
  const {additionalLanguages} = prism;
  // Prism components work on the Prism instance on the window, while prism-
  // react-renderer uses its own Prism instance. We temporarily mount the
  // instance onto window, import components to enhance it, then remove it to
  // avoid polluting global namespace.
  // You can mutate PrismObject: registering plugins, deleting languages... As
  // long as you don't re-assign it
  globalThis.Prism = PrismObject;
  additionalLanguages.forEach((lang) => {
    // eslint-disable-next-line global-require, import/no-dynamic-require
    require(`prismjs/components/prism-${lang}`);
  });
  delete globalThis.Prism;
}

但是手动导出的文件需要手动引入prismjs模块依赖，为了简洁改为直接将复制粘贴了prism-java.js再导入，方法略微有些dirty但是至少可用。