decoder: Improve SignatureAtPos accuracy

[dbanck]

Background

In #135, we added support function signature help.

As Radek mentioned, it doesn't consider the schema, which means that we provide signature help in all contexts/expressions, regardless of whether function calls are expected there or not.

For example, you get help for list or map types in Terraform < 0.15, where functions with the same names exist.

Proposal

• Improve SignatureAtPos to take the schema/outer most expression into account before providing signature help