Add .env vs .env.example key diff check

[vidya381]

Check that all keys defined in .env.example are present in the local .env file.

When should it run?
When both .env.example and .env exist in the project root.

Pass: All keys from .env.example are present in .env

Fail: List the missing keys and tell the user to add them to their .env

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For reference on how to implement a check, look at internal/checks/binary.go. Follow the same pattern.

Once done, register it in internal/check/registry.go.