Provide token modifiers for attributes

[radeksimko]

This is in follow-up to #833

Background

In the above PR we introduce custom semantic token types and modifiers, to enable themes to address configuration parts more accurately and using more familiar names.

Idea no 1: Core vs Provider Attributes

One idea which came up during the implementation - but wasn't implemented - was whether we assign any particular modifiers to attributes. One potential use case would be distinguishing terraform-core from terraform-provider-dependent attributes.

That is e.g. to allow highlighting the following attributes differently

resource "aws_instance" "example" {
  count = 3 # core
  instance_type = "t3.micro" # provider-dependent
}

Idea no 2: Attributes inherit block modifier

We already let nested blocks inherit the parent block modifiers, for example

terraform { # modifier = terraform-terraform
  required_providers { # modifier = terraform-terraform + terraform-requiredProviders
    aws = {
      source = "hashicorp/aws"
    }
  }
}

we could take it further and do the same for attributes

terraform { # modifier = terraform-terraform
  required_providers { # modifier = terraform-terraform + terraform-requiredProviders
    aws = { # modifier = terraform-terraform + terraform-requiredProviders
      source = "hashicorp/aws"
    }
  }
}

The question is where does it become "too granular"? So far we generally avoided assigning provider-specific modifiers, such as provider-aws.

Proposal

TODO - depends on which idea (if any) we go for ^