Single File/Script mode

## Description

Sometimes people open an elixir script and want autocomplete and hover, but this seems to not work correctly at the moment (example #397)

## Notes

- Might partially support this by not starting an engine and allowing AST based features or formatting (things that dont require an engine necessarily), run in a degraded state. Autocompletion might work for standard library.
- This scenario will not have a mix project, need to figure out how that works with our engine.
- Might need another simpler engine runtime implementation that is designed for this purpose
- Scripts can use Mix.install, we need to ensure how we can support that and give autocomplete/definition for those packages.
- might need to parse the script and extract the packages passed to Mix.install, and then install them in a different way, since we don't want to actually execute their script.
- we need to figure out how to cache those packages correctly, we dont want to install them everytime they open the script, but need to correctly cache bust them if they change them.