Add experimental Prefill Required Fields feature

[jpogran]

Needs
hashicorp/hcl-lang#89
hashicorp/terraform-ls#657

Example

[radeksimko]

Can you confirm that this would not require us to delay releasing this option as part of next extension version? LS will actually raise warnings when unknown config options are passed.

Unless it already works that way I think we could avoid sending the option there entirely if it's (default) false, so that it only affects people who explicitly opt-in and do so too early, before upgrading LS.

[jpogran]

Adding to the existing terraform-ls.experimentalFeatures won't send anything automatically because the default value for terraform-ls.experimentalFeatures is undefined. Only when the user adds the property to their config would it send something.

With that in mind, I think this is safe to release at the same time as the LS

[radeksimko]

[github-actions]

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