Provide function signature help

[radeksimko]

Background

LSP provides textDocument/signatureHelp to provide the client with data about a function within the context of a function call.

Terraform configuration language has a number of built-in functions, some of which come from HCL.

Proposal

Function Signature Help

After picking the function name from a completion popup, or typing the name and the "trigger character" ( (opening brace).

Implementation Notes

• Support built-in functions #37
• Introduce SignatureAtPos(filename string, pos hcl.Pos) (*FuncSignature, error) into PathDecoder in hcl-lang/decoder
• Update hcl-lang in LS

[github-actions]

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