Expand all environment variables for Python Executable setting

# Feature Request
## Description
I really don't want to install pyls to my global pip. I'd like to be able to set the python executable to `$HOME/dotfiles/atom/venv/bin/python` (or, even better, `~/dotfiles/atom/venv/bin/python`). When I use $HOME, currently I get three errors:
1. **`ide-python` could not launch your Python runtime.** No Python interpreter found at `$HOME/dotfiles/atom/venv/bin/python`. If you have Python installed please set "Python Executable" setting correctly. If you do not please install Python.
2. **pyls language server for Python unable to start** Error: spawn $HOME/dotfiles/atom/venv/bin/python ENOENT
3. **Unable to start the Python language server.** Make sure to install pyls 0.19 or newer by running:
```sh
$HOME/dotfiles/atom/venv/bin/python -m pip install 'python-language-server[all]'
```

## Related Code
https://github.com/atom-ide-community/ide-python/blob/b8a071b450d9ed24b44f8453bad6e5fbfab40070/lib/utils.js#L24-L60

https://github.com/atom-ide-community/ide-python/blob/b8a071b450d9ed24b44f8453bad6e5fbfab40070/lib/utils.js#L71-L76

## Related Issues
#203, https://github.com/lgeiger/ide-python/issues/182#issuecomment-595830215

Empieza leyendo las secciones referenciadas de lib/utils.js, especialmente las líneas 24-60 y 71-76, para rastrear cómo se usa la configuración Python Executable para iniciar Python y pyls. Se considera completado cuando las variables de entorno como $HOME y ~ se expanden antes de que las comprobaciones del intérprete, los informes de errores y el inicio del servidor de lenguaje utilicen la ruta.

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