Support for .tofu files for opentofu

**Is your feature request related to a problem? Please describe.** When using tfswitch with opentofu (TF_PRODUCT or --product) it will only work if the providers file has an `.tf` file ending. File ending `.tofu` is not supported. **Describe the solution you'd like** When using tfswitch with opentofu the filending `.tofu` should also be taken into account. **Describe alternatives you've considered** Since opentofu supports both file endings, the workaround is to use `providers.tf` instead of `providers.tofu`, even for opentofu projects. **Additional context** Why do i not use `.tf` for opentofu projects? It should be best practice to use destinct file endings for terraform (`.tf`) and opentofu (`.tofu`) projects to prevent wrong tooling. ```sh tfswitch Version: v1.13.0 ```