Plugin crashes in embedded REPL

## Description
The Litegitpuller plugin fails to load when the environment is an embedded Jupyter Lite REPL. It looks like this is happening because the REPL doesn't include the file browser.

It would be great to be able to use the plugin within an embedded REPL as well as a whole.

Just scanning the documentation, this might be possible if we used [ContentsManager](https://jupyterlab.readthedocs.io/en/3.4.x/api/classes/services.contentsmanager-1.html) to access files, rather than IDefaultFileBrowser.

## Reproduce

If Jupyter Lite is hosted on `localhost:8000`, go to `http://localhost:8000/repl/index.html?repo=https%3A%2F%2Fgithub.com%2Fbrichet%2Ftesting-repo&urlpath=notebooks%2Fsimple.ipynb&branch=main`

## Expected behavior
The git repo is fetched and the notebook loaded the same as it is on `http://localhost:8000/index.html`

Browser Output

Plugin '@jupyterlite/litegitpuller:plugin' failed to activate. TypeError: No provider for: @jupyterlab/filebrowser:IDefaultFileBrowser.

    resolveRequiredService index.js:574

    n index.js:453

    activatePlugin index.js:453

    e index.js:494

    activatePlugins index.js:492

    start index.es6.js:248

    T index.js:517

    69610 bootstrap.js:177

    69610 bootstrap.js:148

    Webpack 5

[index.js:497:36](webpack://_JUPYTERLAB.CORE_OUTPUT/node_modules/@lumino/coreutils/dist/index.js)

    e http://localhost:8000/build/jlab_core.b189e4f.js:1

    activatePlugins http://localhost:8000/build/jlab_core.b189e4f.js:1

    start http://localhost:8000/build/jlab_core.b189e4f.js:1

    T http://localhost:8000/build/6584.3a10d08.js:1

    69610 http://localhost:8000/build/repl/bundle.js?_=c3ceefd:1

    69610 http://localhost:8000/build/repl/bundle.js?_=c3ceefd:1

    w http://localhost:8000/build/repl/bundle.js?_=c3ceefd:1

    k http://localhost:8000/build/repl/bundle.js?_=c3ceefd:1

    O http://localhost:8000/build/repl/bundle.js?_=c3ceefd:1

     http://localhost:8000/build/repl/bundle.js?_=c3ceefd:1

     http://localhost:8000/build/repl/bundle.js?_=c3ceefd:1