Use a `.cspell.json` repo config file

Add a `.cspell.json` file in the root dir:

```json
{
  "version": "0.2",
  "language": "en",
  "ignorePaths": [
    ".vscode",
    "**/.git/**",
    "**/vscode-extension/**",
    "megalinter"
  ],
  "words": [
  ]
}
```

Same as we did for the tracker here: https://github.com/torrust/torrust-tracker/issues/111