Provide accurate constraints in index part of references

[radeksimko]

Follow-up from hashicorp/hcl-lang#365 (comment)

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Context

Currently we recognise index expression such as var.foo[var.bar], in the sense that we provide:

• completion
• hover
• semantic tokens
• go-to-definition/references

While this is helpful, we currently treat all [] (indexes) in references as if they were a map, i.e. something indexable with keys of type string.

https://github.com/hashicorp/hcl-lang/blob/f43c27231c1001f4682f6fe72011cdabe692bdef/decoder/expr_any_index.go#L19-L25

Because we check for type convertability and strings/numbers are convertible in cty, this does not impact the functionality in a major way. However, it can make it less helpful in some contexts.

• we're unable to detect invalid or poor practice configuration (e.g. var.map[var.number] or var.list[var.string])
• we're unable to sort completion candidates by prioritising number type for lists/tuples and string type for maps
• the hover data may not be as accurate, or at least not tell the whole story

Proposal

• Ensure constraints for the index [] are inferred from the type of the reference
  • This may require some refactoring as we may not know anything about the type until we have parsed the target of the reference
• Produce diagnostics for invalid or poor practice configuration, e.g. var.map[var.number] or var.list[var.string]
• Sort completion candidates by prioritising number type for lists/tuples and string type for maps
• Display constraint in hover separately from real type #1512