Support relative references / traversals

[radeksimko]

Background

Support for scoped/absolute traversals, where the collection being indexed is a static traversal was added in #485 - this brought support e.g. for aws_instance.example.tags["owner"] or aws_alb.example.access_logs[0].

However Terraform also supports relative traversals, where the collection being accessed is essentially an expression of itself - e.g. a list or a function call

• ["first", "second"][0]
• bar(1, a)[0]
• bar(1, a)["key"]

Language server currently does not support such expressions, which in practice means that e.g.

• no completion is provided within an existing relative traversal, e.g. [ HERE ][0] nor bar( HERE )[0] nor ["first", "second"][ HERE ]
• no data is provided on hover over any expression within such relative traversal expression (neither the collection nor key)
• no tokens are highlighted within such relative traversal expressions

Proposal

Completion

TODO

Hover

TODO

Semantic Tokens

TODO