Add option for supported language

[ksss]

There are several RuboCop extensions that target non-Ruby files.

• https://github.com/rubocop/rubocop-md
• https://github.com/r7kamura/rubocop-erb
• https://github.com/r7kamura/rubocop-slim

However, since VSCode is not informed that this extension should activate for files like markdown or erb, diagnostics and auto-corrections cannot be displayed in VSCode.

This change allows you to add languages that can be used in VSCode settings, enabling the extension to be used for these languages.

Settings screen shot

Example for erb

[ksss]

I also has released https://github.com/ksss/rubocop-on-rbs .
It needs language support for rbs.
I would appreciate it if you could consider this change.

[ksss]

@koic
This change is still useful to me today and I would love to share it with other users.
Would you mind sharing your current thoughts on this change?