Add support for tfvars (variable files)

[beandrad]

So that we can use the language server for "tfvars" files.

Closes #50

[hashicorp-cla]

All committers have signed the CLA.

[radeksimko]

Thank you for the PR, this looks like a good start! I left you some in-line comments.

I have yet to decide about the language ID to use for these files but keep it tfvars for now - I assume it's just a minor implementation detail anyway that can be changed easily.

[radeksimko]

This is starting to look almost ready for merge 👍🏻

Aside from my in-line comments - can you also add some E2E tests here?

• https://github.com/hashicorp/terraform-ls/blob/main/internal/langserver/handlers/complete_test.go
• https://github.com/hashicorp/terraform-ls/blob/main/internal/langserver/handlers/hover_test.go
• https://github.com/hashicorp/terraform-ls/blob/main/internal/langserver/handlers/semantic_tokens_test.go
• https://github.com/hashicorp/terraform-ls/blob/main/internal/langserver/handlers/formatting_test.go

[radeksimko]

Aside from the last two comments this LGTM! Thank you for all your effort to get it to this point @beandrad

I will have a discussion internally about what the right language ID is for tfvars - if not tfvars and then we can merge it.

[radeksimko]

I did some more thorough testing in VS Code as well and I just noticed two more things that we should address here:

• symbols are not reported for tfvars, I reckon there are some minor changes needed in

  terraform-ls/internal/langserver/handlers/symbols.go

  Lines 40 to 43 in adf5e7d

  	d, err := decoder.DecoderForModule(ctx, mod)
  	if err != nil {
  	return symbols, err
  	}

• document links handler should probably just completely ignore tfvars: https://github.com/hashicorp/terraform-ls/blob/adf5e7d22fe483c72c7209dee8ee90868738f123/internal/langserver/handlers/document_link.go - I guess it's no-op anyway, but better to be explicit about it

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