Fix prefixing unused variables with underscore

[sheldak]

Since I noticed some issues with this feature and an attempt to revert it (#775), I would like to propose a fix that hopefully makes it more usable.

Instead of adding _ at the beginning of the line, I would replace the unused variable var with _var using String.replace/4 with the following regex:

"(?<![[:alnum:]._])#{variable}(?![[:alnum:]._])"

I assume the unused variable does not have any of

• .
• _
• letters or numbers

as a prefix or suffix.

Also, to prevent replacing incorrectly in the cases like:

def foo do
  var1 = Enum.map([1, 2], fn var1 -> var1 + 1 end) # we don't want to replace inner `var1` 
  var2 = " var2 " # we don't want to replace the string `"var2"`
end

I'm not creating a quickfix if there is more than one match in the line.

[lukaszsamson]

use Regex.escape/1 on variable. Elixir variables can have non regex friendly chars !?.

[lukaszsamson]

Thanks for the PR but this will need to be solved properly by AST manipulation