Fixes rubocop#6132.

### Summary

This PR fixes a false negative for `Naming/FileName`
when `Include` of `AllCops` is the default setting.

The important change in this PR is below.

```diff
diff --git a/lib/rubocop/cop/naming/file_name.rb
b/lib/rubocop/cop/naming/file_name.rb
index a025a73b7..be3f1c5aa 100644
--- a/lib/rubocop/cop/naming/file_name.rb
+++ b/lib/rubocop/cop/naming/file_name.rb
@@ -32,7 +32,7 @@ module RuboCop

         def investigate(processed_source)
           file_path = processed_source.file_path
-          return if config.file_to_include?(file_path)
+          return if config.file_to_exclude?(file_path)
```

The problem is that the target to be excluded was
`Include` instead of `Exclude`.

Also this PR adds the `RuboCop::Config#allowed_camel_case_file?`
method to judge ignoring `Gemfile`, `Rakefile`, etc
described in` Include`.

### Other Information

This false negative was noticed by adding `**/*.rb` to
config/default.yml at rubocop#5882.

https://github.com/rubocop-hq/rubocop/pull/5882/files#diff-e93280b3b31a6438c533a5f3232340d8R18