[Proposal] Added fn::final built-in to prevent overriding values

[tehsis]

This pr adds support for fn::final built-in function which prevents overriding properties defined on a parent environment.

eg.

env proj/base:

values:
  foo:
    fn::final: 4

env proj/child:

imports:
  - proj/base
values:
  foo: 5

results in
Error: cannot override final value