Fix highlighting comparable method definition (#2149)

Previously, when trying to highlight the definition of the comparable
method (aka the spaceship, `<=>`), the `<` would be highlighted as
`Name::Function` but the `=>` would be `Operator`. This happens because
of the ordering of potential matches in the `funcname` regular
expression:

```ruby
%r(
  # snip
  <<? | >>? | <=>? # snip
)
```

The method name is always caught by the single `<` match, and so never
makes it to the `<=>` possibility.

This commit fixes the issue by trying to match `<=>?` before `<<?`. This
ensures it _can_ be matched, and since none of the following potential
matches are a superset of its (`<=>?`) matches it won't cause any issue
with subsequent possibilities.

Author: skipkayhil

lib/rouge/lexers/ruby.rb

@@ -297,7 +297,7 @@ def self.detect?(text)
           (
             [\p{L}_]\p{Word}*[!?]? |
             \*\*? | [-+]@? | [/%&\|^`~] | \[\]=? |
-            <<? | >>? | <=>? | >= | ===?
+            <=>? | <<? | >>? | >= | ===?
           )
         )x do |m|
           puts "matches: #{[m[0], m[1], m[2], m[3]].inspect}" if @debug

spec/lexers/ruby_spec.rb

@@ -89,6 +89,20 @@
           ['Literal.Number.Float', '20.4e+8']
       end
     end
+
+    describe 'method definition' do
+      it 'identifies comparable method' do
+        assert_tokens_equal "def <=>(o); end",
+          ["Keyword", "def"],
+          ["Text", " "],
+          ["Name.Function", "<=>"],
+          ["Punctuation", "("],
+          ["Name", "o"],
+          ["Punctuation", ");"],
+          ["Text", " "],
+          ["Keyword", "end"]
+      end
+    end
   end
 
   describe 'guessing' do