Ask user to download HIE if not found

[Tehnix]

I completely missed that HIE now has binaries for macOS, which (for me) means that we can finally get some sleak UX for automatically fetching the needed HIE binaries. I'm pretty sure there's some pre-built for Linux also, but can't seem to find them atm.

The flow could go something like:

           Try to detect if HIE is on path
                         |
                         v
   If not: ask user to download HIE. If it is: carry on as usual
                         |
                         v
   The user is presented with the dialog and has the option of
   "Download HIE", "Dismiss", "Permanently Dismiss"
                         |
                         V
   If "Download HIE" is chosen, we fetch HIE and put it on path

I think this will help majorly w.r.t. adoption, since now people can just install the extension and get running almost immediately, without spending an hour (almost literally) compiling the ~6 different HIE versions.

[expipiplus1]

Looks as though this is waiting for hie to be on hackage/stackage: https://github.com/alanz/vscode-hie-server/blob/master/src/extension.ts#L75

[jneira]

In addition to upload to hackage/stackage we want to add precompiled binaries: haskell/haskell-ide-engine#1068
There is a similar issue in hie: haskell/haskell-ide-engine#1383 so i will close this one if you dont mind (reopen if it is not the case)