Add haskell filetype support for use with ghci

This adds a haskell ftplugin that implements the `_EscapeText_haskell` function to correctly send
haskell code to a running `ghci` session in `tmux`.

Makes use of the `:{` and `:}` operators in `ghci` to send multiline blocks and also prepends `let ` when needed (eg. in a function definition, because `ghci` requires this but it is not required in a haskell file).