Raise warnings for unsupported Terraform versions

[radeksimko]

Current Version

v0.3.2

Use-cases

The language server supports a limited range of terraform versions, generally >=0.12. The upper bound preventing folks from using it with 0.13 was removed as part of #149 but the language server still relies on some implementation details of Terraform (such as files under .terraform) which may not provide stability guarantees. This means that user may be unaware that they're using the language server with unsupported Terraform version.

Attempted Solutions

Raise error - which artificially prevents users from using higher Terraform versions which may in fact work.

Proposal

We can warn users when they attempt to use unsupported version e.g. by publishing warning as part of initialize.