[.github] Add helper to return in-memory "model" of spec folder

[mikeharder]

• Fixes [EngSys] Add API readme.js:parse() to return all contained data #33227

I believe the authoritative source for readme.md parsing is package @autorest/configuration. I don't think we'd want to use this implementation directly, but we can use it as a reference, or possibly in tests to ensure our new implementation matches the existing logic.

Tests that might show how to use the API:

https://github.com/Azure/autorest/blob/53c9a2b82e10fed62097337bbdc516aed4120d49/packages/libs/configuration/test/scenarios.test.ts

https://github.com/Azure/autorest/blob/53c9a2b82e10fed62097337bbdc516aed4120d49/packages/extensions/core/test/configuration.e2e.ts

[openapi-pipeline-app]

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