vscode: implementing "predicate" language

[flyinghermit]

This issue is to discuss whether we should implement a new language support for predicate policies.

Since predicate policy files are python code and python files, all other installed extensions that are registered for python language also gets activated while authoring policy files. This result in conflicting IntelliSense including auto suggestion, code completion etc. For optimal user experience, we want only predicate extension to be active while working with policy files. Unfortunately, VS Code does not offer easy way to activate/deactivate other installed extensions, without going through manual configuration update, and in some cases, needing to disable extensions altogether.

The solution to this issue is to implement our own language type as "predicate". This will improve overall user experience by:

• Keeping out other python extensions, avoiding conflicting IntelliSense in predicate policies.
• Prevent predicate extension mangling with other python projects.

But supporting new language comes with its own caveat: We will also need to implement syntax highlighting, go-to functionality, etc. Given the scope of predicate policy files, we may not need to implement all the IntelliSense that official python extension supports but this will be something that may take substantial effort. And this needs to be repeated again when we support other programming languages in future besides python.