goreleaser: Enable pre-release publishing

[radeksimko]

TODO:

• Verify impact on https://github.com/hashicorp/vscode-terraform (earlier versions which do not bundle LS)
• Docs to cover expectations of pre-releases and how to obtain them
• Linux packages - do other product publish them for pre-releases? Do we need to do anything extra?

---

Closes #456

See relevant docs at https://goreleaser.com/customization/release/#github

  # If set to auto, will mark the release as not ready for production
  # in case there is an indicator for this in the tag e.g. v1.0.0-rc1
  # If set to true, will mark the release as not ready for production.
  # Default is false.
  prerelease: auto

and https://goreleaser.com/customization/homebrew/

    # If set to auto, the release will not be uploaded to the homebrew tap
    # in case there is an indicator for prerelease in the tag e.g. v1.0.0-rc1
    # Default is false.
    skip_upload: true

[jpogran]

We specify latest by default using https://github.com/hashicorp/vscode-terraform/blob/ddddaf39c5970266a2d5ea21c75e573e31cb5152/src/installer/updater.ts#L36. DEFAULT_LS_VERSION is defined at https://github.com/hashicorp/vscode-terraform/blob/v2.19.0/src/installer/detector.ts#L6. This means the default action is to download only the latest stable version.

I verified this by setting up a test repo with the methods we use and pointed it to the terraform project which produces pre-release versions.

A user would have to manually specify a pre-release version using the deprecated terraform.languageServer.requiredVersion to purposely download a pre-release version.

[radeksimko]

I will cover docs update in a separate PR and I confirmed that Linux packaging for pre-releases is handled and we don't need to do anything extra for that.

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