Bad Label: Missing `[` identified as an "unmatched end

[schneems]

After #83 this code:

def foo
  1,2,3,4]
end

Produces this incorrect label:

DeadEnd: Unmatched `end` detected

This code has an unmatched `end`. Ensure that all `end` lines
in your code have a matching syntax keyword  (`def`,  `do`, etc.)
and that you don't have any extra `end` lines.

file: /tmp/scratch.rb
simplified:

      1  def foo
    ❯ 2    1,2,3,4]
      3  end

Known syntax error variations:

Here are some syntax errors that Ruby will generate for various code:

$ cat <<EOF | ruby -wc
def foo
  1,2,3,4]
end
EOF
-:2: syntax error, unexpected ',', expecting `end'
  1,2,3,4]

$ cat <<EOF | ruby -wc
def foo
  foo = 1,2,3,4]
end
EOF
-:2: syntax error, unexpected ']', expecting `end'
  foo = 1,2,3,4]

Fix

This is related to #68 because missing a leading bracket seems to be a general special case. Ideally, we tell the user that they're missing the bracket. If we can't do that, maybe we can avoid outputting an incorrect message.