ruby-mode: Support endless singleton method definitions too

* lisp/progmodes/ruby-mode.el (ruby-endless-method-head-re):
Update to match not only 'self.' but 'xyz.' as well (bug#54702).

* test/lisp/progmodes/ruby-mode-resources/ruby.rb (Bar#foo=):
Update example.

lisp/progmodes/ruby-mode.el

@@ -135,7 +135,8 @@ This should only be called after matching against `ruby-here-doc-beg-re'."
   "Regexp to match symbols.")
 
 (defconst ruby-endless-method-head-re
-  (format " *\\(self\\.\\)?%s+[?!]? *\\(([^()]*)\\)? +=" ruby-symbol-re)
+  (format " *\\(%s+\\.\\)?%s+[?!]? *\\(([^()]*)\\)? +="
+          ruby-symbol-re ruby-symbol-re)
   "Regexp to match the beginning of an endless method definition.
 
 It should match the part after \"def\" and until \"=\".")

test/lisp/progmodes/ruby-mode-resources/ruby.rb

@@ -533,6 +533,8 @@ def foo=(
         baz,
         bar
       )
-    hello
+    def baz.full_name = "#{bar} 3"
+
+    baz
   end
 end