Detect and integrate with tfenv

[radeksimko]

Current Version

0.9.0

Use-cases

Users with tfenv can face challenges due to the way tfenv works when executing terraform commands.

e.g. as demonstrated in #151 (comment) tfenv will automatically install Terraform of a particular version per .terraform-version in PWD and the output doesn't match the output of typical terraform version, so terraform-exec is unable to parse that successfully.

Proposal

• Understand UX and user expectations about how the integration should work, e.g.
  • Should tfenv be installing Terraform as part of initialization?
  • Should that happen silently, or would you expect a popup and confirm the installation?
  • Should LS be always passing ENV var which prevents tfenv from automatically installing anything and should it be left to the user to manage outside of LS?

Note: The work may also involve terraform-exec as that's the library LS uses for executing Terraform.

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Related: hashicorp/terraform-exec#6