Suppress `vars` context validation in an environment set via an expression.

Problem:
I am using a lot of repository environment variables, access via the vars context. The plugin is raising "Context access might be invalid:" warnings on every line which makes it difficult to spot actual issues and is making me sad.

Proposed solution:

I think that this functionality isn't relevant for vars/secrets contexts because there is no way for them to be discernible by the plugin.

It would be great to be able to configure the plugin to ignore these warnings in some circumstances (such as by a string pattern or by check boxes for various contexts. Or at least to be able to turn it off globally.

**Additional context**
Related to
[https://github.com/github/vscode-github-actions/issues/67][https://github.com/github/vscode-github-actions/issues/47]

Screenshot of my minimap to show you what I'm dealing with, all the yellow lines are this warning!


Start by reproducing the vars-context validation warnings described in the issue and read the linked issues #67 and #47 for existing decisions. Identify where validation emits these warnings and define a configurable suppression behavior; done means the selected warnings no longer appear while other context warnings remain visible.

索引モデルが issue の本文から書いたものです。