Recognize .resource files as Robot Framework files (#2158)

**Problem**

RobotFramework supports `.resource` files, but they aren’t highlighted
currently.

See documentation:
https://robotframework.org/robotframework/latest/RobotFrameworkUserGuide.html#resource-files

**Solution**

Extend the RobotFramework lexer to detect `.resource` files.

Author: 304

lib/rouge/lexers/robot_framework.rb

@@ -10,7 +10,7 @@ class RobotFramework < RegexLexer
       title "Robot Framework"
       desc 'Robot Framework is a generic open source automation testing framework (robotframework.org)'
 
-      filenames '*.robot'
+      filenames '*.robot', '*.resource'
       mimetypes 'text/x-robot'
 
       def initialize(opts = {})

spec/lexers/robot_framework_spec.rb

@@ -8,6 +8,7 @@
 
     it 'guesses by filename' do
       assert_guess :filename => 'foo.robot'
+      assert_guess :filename => 'foo.resource'
     end
 
     it 'guesses by mimetype' do