PartialInstanceVariable is great for partials, but I'd like to forbid instance variables in all templates.

The PartialInstanceVariable linter implementation has this guard to restrict it to partials:

https://github.com/Shopify/erb-lint/blob/c30fa2783d5a028b9c67795ab20f4d6b19280b9e/lib/erb_lint/linters/partial_instance_variable.rb#L11

So a linter simply without this restriction would work for my case.

The only thing between me and a PR is how to structure the configuration. Some options I came up with:

1. Create a new InstanceVariable linter that works for all templates and document the fact that if you enable this and PartialInstanceVariable, you may get double warnings
2. Create a new NonPartialInstanceVariable linter that only works on non-partials to avoid double linting if someone has both enabled
3. Add configuration to PartialInstanceVariable of the sort all_templates: true to make it work for all templates
4. Deprecate PartialInstanceVariable and create a InstanceVariable linter with a partials_only: true configuration

I'm inclined toward option 1 for simplicity, but I'm very open to whatever so I can add this linter 😄