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)
```

Start by inspecting lib/index.js, the module path shown in the TypeScript error, to identify the exported API, including github.getFlatConfigs. Add a declaration file that represents those exports, then verify that the TypeScript language server no longer reports the missing declaration and can suggest getFlatConfigs.

Written by the indexing model from the issue text.