Add experimental Prefill Required Fields feature

[jpogran]

This adds a new experimental setting for prefilling required fields as blocks are completed from intellisense. For example, completing a resource provider will fill in the required attributes automatically.

Needs hashicorp/hcl-lang#89

[radeksimko]

LGTM. I reckon you'll want to re-pin to the merged revision though, once hashicorp/hcl-lang#89 is merged.

[radeksimko]

Oh, one last thing - I think we should also document this new experiment here:

https://github.com/hashicorp/terraform-ls/blob/main/docs/SETTINGS.md#experimentalfeatures

It doesn't have to be part of this PR, but we should do it before releasing it, so that we can also link to it from the changelog.

[jpogran]

Updated the pin for hcl-lang, and added detail for the new setting to SETTINGS.md

[radeksimko]

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