Add ability to provide an executable instead of downloading it

[LamprosPitsillos]

Watching buffer: 2                                                                            
Could not start dynamically linked executable: /home/inferno/.local/share/nvim/typst-preview/t
ypst-preview-linux-x64                                                                        
NixOS cannot run dynamically linked executables intended for generic                          
linux environments out of the box. For more information, see:                                 
https://nix.dev/permalink/stub-ld       

Was getting the above error on NixOs , so i decided to add a way to provide your own executable Closes #15

[LamprosPitsillos]

I tried to break it but seems to be working as expected , please let me know of any other changes needed!

[chomosuke]

How does this change handle the "not updating when bin is provided" part that i implemented in the loop implementation?

download_bin() already calls up_to_date() which will return true if the bin has been provided.

I agree that in that case the messages are not ideal and I will fix that soon.

[LamprosPitsillos]

Other than that slight clarification in the README and docs about the path in dependencies_bin , everything LGTM! Thanks for merging!