Add version output

[radeksimko]

This is likely to conflict with #23 - I can rebase the other one when either of these PRs are merged.

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It is planned to parse the JSON output in terraform-exec at some point as per hashicorp/terraform-exec#13 and this PR would therefore support these efforts.

The struct is practically copy&paste of the one used in core:

https://github.com/hashicorp/terraform/blob/ae025248cc0712bf53c675dc2fe77af4276dd5cc/command/version.go#L26-L32

I just added omitempty to platform since that was introduced in v0.15, although I don't expect anyone will actually use these structs for marshalling.

[vancluever]

@radeksimko I've merged your other two PRs, wanna fix this one so that it's ready to be merged?

PS: as there's possible talk about integrating this with some other core repos, I'm going to just hold off on tagging a new version for now unless it's holding up work. I'll leave it to you though to see what you want to do. Let me know!

[radeksimko]

@vancluever Rebased.

as there's possible talk about integrating this with some other core repos, I'm going to just hold off on tagging a new version for now unless it's holding up work. I'll leave it to you though to see what you want to do. Let me know!

I am aware. We've been briefly chatting about the transition and I think we'd prefer to still cut at least one more release (0.7.0 presumably), so we can use it in hashicorp/terraform-exec#68 (just for consistency really) which in turn unblocks hashicorp/terraform-ls#323

[vancluever]

Sounds good. I'll tag shortly!