Fix SpaceAroundOperator error messages for multiline expressions

We should not display multiline expressions in `scss-lint`'s single line
output format. Condense messages into a single line.

lib/scss_lint/linter/space_around_operator.rb

@@ -4,15 +4,15 @@ class Linter::SpaceAroundOperator < Linter
     include LinterRegistry
 
     def visit_script_operation(node) # rubocop:disable Metrics/AbcSize
-      source = source_from_range(node.source_range).chop
+      source = normalize_source(source_from_range(node.source_range))
       left_range = node.operand1.source_range
       right_range = node.operand2.source_range
 
       # We need to #chop at the end because an operation's operand1 _always_
       # includes one character past the actual operand (which is either a
       # whitespace character, or the first character of the operation).
-      left_source = source_from_range(left_range).chop
-      right_source = source_from_range(right_range).chop
+      left_source = normalize_source(source_from_range(left_range))
+      right_source = normalize_source(source_from_range(right_range))
       operator_source = source_between(left_range, right_range)
       left_source, operator_source = adjust_left_boundary(left_source, operator_source)
 
@@ -58,6 +58,11 @@ def source_between(range1, range2)
                                                 range1.importer))
     end
 
+    # Removes trailing parentheses and compacts newlines into a single space
+    def normalize_source(source)
+      source.chop.gsub(/\s*\n\s*/, ' ')
+    end
+
     def adjust_left_boundary(left, operator)
       # If the left operand is wrapped in parentheses, any right parens end up
       # in the operator source. Here, we move them into the left operand