textDocument/complete: Complete resource+datasource names

[radeksimko]

Until hashicorp/terraform#24261 is resolved we'd only be able to complete resource names of providers which are already declared in the config.

That seems like a reasonable temporary limitation though, so it's still worth implementing even without hashicorp/terraform#24261

Example

provider "aws" {
  region = "us-west-2"
}

resource "<HERE>" "" {

}

We should also decide out whether/how to complete incomplete declarations, such as

provider "aws" {
  region = "us-west-2"
}

resource "<HERE>

I'm not sure how HCL parser behaves in this case ^

[radeksimko]

Completing incomplete declarations is tracked under #57

[ghost]

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