In fsi, support `#exit;;` as an alias to `#quit;;`

[T-Gro]

As a non-native speaker, I easily make a mistake when spelling the word quit.
I also tend to forget which is the right sequence of characters for quitting the dotnet fsi console, putting me in a vim-like stage of not being able to shutdown the console.

I am not proposing to change the default, but I think we should support #exit;; as well, side-by-side existign #quit;; instruction.

One of the reasons for exit is that it is a term used in dotnet and compiled F# applications already.

[Martin521]

One of the reasons for exit is that it is a term used in dotnet and compiled F# applications already.

And in shells.

BTW exit 0;; already works.