Support provider-defined functions syntax

[dbanck]

• Part of Provider-defined functions support vscode-terraform#1674

Background

We currently only have a static list of valid function names in the Terraform grammar. Provider-defined functions introduce a new syntax for function calls, provider::aws::noop(). Also, it's impossible to compile all function names as a static list in the grammar, so we'll just assume that any function call with this new snytax is valid.

Proposal

• Update Terraforms functions grammar block, e.g.

  functions:
    begin: ([:\w]+)(\()
    name: meta.function-call.hcl
    comment: Built-in and provider defined function calls
    beginCaptures:
      "1":
        patterns:
          - match: \b(abs|abspath|...|zipmap)\b
            name: support.function.builtin.terraform
          - match: \b(provider::\w+::\w+)\b
            name: support.function.provider
      "2":
        name: punctuation.section.parens.begin.hcl
    end: \)

• Add some tests for the new syntax