completion: Prompt picking type of provider/data/resource automatically

[radeksimko]

Depends on hashicorp/hcl-lang#5
Closes hashicorp/vscode-terraform#373

This enhances the UX around completion of provider, data source and resource completion by automatically triggering the suggestion box on the client. This accounts for the fact that people expect on-type completion to work throughout the document and it doesn't work in labels due to the fact that quotes trigger some kind of "string literal mode" in some editors, such as VS Code. See more at hashicorp/vscode-terraform#373

It may feel like a hack, but FWIW the Dart language server and the Scala one use the special command name, which seems registered by VS Code and also Sublime Text at this point, but I wouldn't be surprised if other editors/plugins followed the suit.

microsoft/language-server-protocol#565 tracks a related LSP feature.

Before

(label completion had to be manually triggered via Ctrl+Space)

After

(label completion triggered automatically)

[ghost]

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