Move document encoding code from `hashicorp/terraform-ls` to `hashicorp/hcl-lang`

[rcjsuen]

This library operates off of byte offsets instead of lines and columns which is the basis of many LSP requests. This effectively means that anyone adopting this library will have to calculate byte offsets before calling functions such as HoverAtPos.

From what I can tell, hashicorp/terraform-ls has internal/document/position.go and internal/lsp/position.go that performs these conversions from LSP types. Are there any plans to move those functions over here to hashicorp/hcl-lang to make people's lives easier?