Format on save code action

[jpogran]

This adds a code action to format a file based on the editor.codeActionsOnSave setting. This can either be a global setting or one specific to the terraform language.

Fixes #327

[hashicorp-cla]

All committers have signed the CLA.

[radeksimko]

I gave it spin and it worked nicely in both VS Code and Sublime Text. 👍🏻

Aside from my inline comments:

We should also add an RPC-level test for this new handler, similar to the one we have in

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

Line 33 in a7089c3

func TestLangServer_formatting_basic(t *testing.T) {

and perhaps one for tfvars

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

Line 194 in a7089c3

func TestLangServer_formatting_variables(t *testing.T) {

I'd probably omit the version check as it's already heavily tested in multiple places, including terraform-exec itself.

Lastly it would be great to get this new feature documented. The best way to do so is probably adding Code Actions section similar to Code Lens we have here https://github.com/hashicorp/terraform-ls/blob/main/docs/language-clients.md#code-lens and also briefly explaining the expected use case for the two kinds (source.formatAll and source.formatAll.terraform-ls) - one serving the needs of a client which has languageID-based configuration and the other one client which has global configuration (such as Sublime Text).

[radeksimko]

I left you just some very minor inline suggestions, but the PR looks pretty good in general and ready for merging.

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