Check and constrain Terraform version

[radeksimko]

As the language server relies on Terraform output it should define a range of versions it supports and provide reduced functionality if unsupported version is found - e.g. only offer autocomplete candidates we can without knowing the schema.

[radeksimko]

We may also want to start versioning internal/terraform/lang per Terraform versioning scheme to ensure we always provider the language features according to the version of Terraform used in the config.

This however shouldn't be necessary for the first release as we are only auto completing items from schema, which is pulled from Terraform itself.