Replace coreFunctions with the new functions merger

[dbanck]

• Part of Provider-defined functions support vscode-terraform#1674
• Depends on Add support for provider-defined functions terraform-schema#320

Background

We currently support all built-in core functions as static definitions in terraform-schema. The provider functions will be part of the provider schema JSON and should be parsed whenever we get a provider schema (bundled or via CLI). Both need to be merged into a single list of function signatures.

Proposal

• Replace the existing coreFunctions helper with a new func functionsForModule(mod *state.Module, schemaReader state.SchemaReader) (map[string]schema.FunctionSignature, error) helper
• Use the recently introduced tfschema.ResolveVersion helper, to obtain the Terraform version
• Use the new functions merger to combine the static list of built-in functions with the provider-defined one's

[github-actions]

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