Cut out Terraform rules into a ruleset plugin

[wata727]

See also #1056

Terraform rules have been provided as built-in rules that are different from other rules for historical reasons.
https://github.com/terraform-linters/tflint/tree/642c99c08c282dd43ea10ebe5da16eb9f2a3252b/rules/terraformrules

By cutting them out as a ruleset plugin, like the AWS ruleset, I would like to reduce internal Terraform package dependencies. I believe that it will have a positive impact on users as it allows Terraform rules to be treated like any other ruleset.

Whether or not to make the Terraform ruleset a bundled plugin is worth considering. It's a good idea to first copy the existing rules to another repository and provide it as a bundled plugin. After that, I'm not sure if it's better to remove it and let a user install it explicitly, or leave the plugin bundled.

In designing the UX, ESLint's design will be helpful. ESLint has a config like "eslint:recommended", and it may be good to group the existing rules as well.
https://eslint.org/docs/rules/

This requires ongoing investigation, but I think there is still room for improvement in the first experience of TFLint. I would like to explore it in this issue.