Add support for function signature help

[dbanck]

This PR enables signature help for function signatures.

To make this work, we first add the function signatures from terraform-schema (introduced in hashicorp/terraform-schema#145) to the PathContext. This work is also required to enable completion, hover, etc. for functions later on.

We then add a new handler for the textDocument/signatureHelp LSP method.

UX

Nested function calls with variadic parameters

Functions without any arguments

No signature help, if passing too many arguments

Multiline function calls

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• Depends on Add support for built-in functions terraform-schema#145
• Depends on Add support for SignatureAtPos hcl-lang#135
• Part of Support built-in functions #37
• Closes Provide function signature help #857

[radeksimko]

LGTM overall, works great 🥇 👍🏻

I left only very minor questions and comments in-line but they're not really blocking.

[github-actions]

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