Add TypeScript declaration file

In order to make the TypeScript language server see what actually is inside `eslint-plugin-github`, and to correctly suggest `github.getFlatConfigs`, I would like to request the addition of a declaration file.

```console
Could not find a declaration file for module 'eslint-plugin-github'. '/Users/robin/Sites/eslint-config-sweet/node_modules/eslint-plugin-github/lib/index.js' implicitly has an 'any' type.
Try `npm i --save-dev @types/eslint-plugin-github` if it exists or add a new declaration (.d.ts) file containing `declare module 'eslint-plugin-github';`ts(7016)
```

先檢查 lib/index.js，也就是 TypeScript 錯誤中顯示的模組路徑，以識別匯出的 API，包括 github.getFlatConfigs。新增一個表示這些匯出的宣告檔案，然後驗證 TypeScript 語言伺服器不再回報缺少宣告，並且可以建議 getFlatConfigs。

由索引模型根據 Issue 內容生成。