Add Support for traversal expressions

[radeksimko]

The decoder currently can't provide any LSP functionality related to expressions.

For example in Terraform:

data "aws_ami" "ubuntu" {
  most_recent = true

  filter {
    name   = "name"
    values = ["ubuntu/images/hvm-ssd/ubuntu-focal-20.04-amd64-server-*"]
  }

  filter {
    name   = "virtualization-type"
    values = ["hvm"]
  }

  owners = ["099720109477"] # Canonical
}

resource "aws_instance" "web" {
  ami           = HERE # <---
  instance_type = "t3.micro"
}

Currently all we "prefill" for ami is a static "value", but the data block's attributes above can be referred to as data.aws_ami.ubuntu.attr_name.

The schema will need to know how blocks/attributes are referenced to and be able to lookup references of a particular type, so that we can e.g. offer data.aws_ami.ubuntu.id (knowing it refers to a string) for ami (string) field.

Also references can be type-less and in such case we may need to filter references based on the "namespace", e.g.

provider "aws" {
  alias = "uswest"
}

resource "aws_instance" "web" {
  provider = HERE # <--
}