Fix: Infix holes no longer filled using prefix notation

[OliverMadine]

fix: #1690

In follow up to my response to @July541's PR (#1694), I have written a different solution to cover the cases of filling holes with infix operators.

applying the "replace _ with (+)" code action to

test :: Int -> Int -> Int
test a1 a2 = a1 `_` a2

now results in

test :: Int -> Int -> Int
test a1 a2 = a1 + a2

rather than

test :: Int -> Int -> Int
test a1 a2 = a1 `(+)` a2

Note: This solution depends on processHoleSuggestions returning infix functions in their postfix form (e.g. (+)) to determine if a function infix/postfix. I imagine there might be a nicer way of doing this.

[isovector]

Sorry, merged master into the wrong PR! My bad!

[OliverMadine]

Closed due to GitHub issues (committed to branch master by mistake). I will reopen shortly (#1708)