Input key checks are unnecessarily case-sensitive

**Describe the bug**
There are a lot of contexts whose possible input keys are case insensitive. For example, both `secrets.BANANA` and `secrets.banana` work just fine (from the [github secrets docs](https://docs.github.com/en/actions/security-guides/encrypted-secrets#naming-your-secrets): "Names are not case-sensitive."). This applies to secrets, with blocks, and many more. The extension currently erroneously reports an error when not using the same casing as the definition

**To Reproduce**
Steps to reproduce the behavior:
1. Create repo secret called "BANANA"
2. Use secret via secrets.banana

**Expected behavior**
No error is reported

**Screenshots**

**Extension Version**
`v0.25.4`

**Additional context**
-

Start by locating the extension's validation logic for input keys, especially references to secrets and with blocks. Reproduce the reported case with a definition named BANANA and a reference to secrets.banana; done means no error is reported for case differences in supported case-insensitive contexts.

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