Merge #6885

Merge #6885

6885: Add !language-configuration.json to .vscodeignore r=matklad a=bryangingechen

#6840 added a `language-configuration.json` file to `package.json`, but because `.vscodeignore` was not updated, this file was not uploaded when the extension was published, leading to constant errors in the VS Code dev tools console:
```
[Extension Host] stack trace: Error: ENOENT: no such file or directory, open '/Users/-/.vscode/extensions/matklad.rust-analyzer-0.2.416/language-configuration.json'
```

Co-authored-by: Bryan Gin-ge Chen <bryangingechen@gmail.com>