Improve highlighting of built-in functions

[dbanck]

The current version of the Terraform language grammar contains a list of built-in functions. However, as we don't know which version of Terraform a user uses, it is highly possible that the list is either incomplete or contains removed functions.

https://github.com/hashicorp/vscode-terraform/blob/562c7cd7b27b5599b4fb1d4ae21b4fc63639dd91/syntaxes/terraform.tmGrammar.json#L750

To improve the syntax highlighting of built-in functions, we should remove the static list from the grammar file and use semantic token highlighting via the LS. The language server has a better understanding of which Terraform version is used and which functions are available in this version.

[radeksimko]

Just wanted to add (as @jpogran proposed) that in addition to semantic token highlighting in LS we could also consider generating the grammar from a list of functions which would provide convenient way of highlighting correct function names.

This means however that we'd need to - (1) have the list of functions accessible somewhere (see hashicorp/terraform-ls#37 and/or TF-418), and (2) always reflect only the latest version of Terraform language in the grammar.

We could do something similar for variable types - with the same caveats as above.

That is not to propose this needs to be addressed together but just to add some context for posterity.