Consider providing hover data for map keys

[radeksimko]

Context

As part of #496 and hashicorp/hcl-lang#199 in particular, it was decided to not provide any hover data for map items, except for the value/expression. This is in contrast to what we do for objects.

It is unclear whether this [lack of] data is actually useful to users.

object

map

Proposal

We could provide data which either represents the whole map, i.e. the same we provide when hovering over the empty space or {} braces

or treat the map key similarly to object attribute.