Broken `terraform_remote_state` backend completion

[radeksimko]

Context

This "broke" during the big reshuffle made as part of hashicorp/hcl-lang#177 and hashicorp/hcl-lang#187. While from continuity perspective this could be seen as a bug, it was a decision to avoid the complexity - esp. related to handling of multi-line strings and leave out the functionality entirely.

So - from terraform-ls perspective this is a bug, but from hcl-lang perspective it's a missing feature.

Expected Behaviour

• Completing just "" on attribute completion and triggering completion again
• Bring all available backend names on expression completion

Proposal

We could pick one of the two routes:

• Implement completion for LiteralValue{} of string type, similarly to how we already implement it for boolean: https://github.com/hashicorp/hcl-lang/blob/0ab4d92a142f208f9ce72760818c357e9ddeb850/decoder/expr_literal_value_completion.go#L49-L52
• schema/decoder: Replace LiteralValue with LiteralType's Enum hcl-lang#260 and then implement completion based on that (which should be slightly easier)