[TypeSpec Validation] Determine specs to validate by following imports

[mikeharder]

Currently, Get-TypeSpec-Folders.ps1 validates all specs under folder specification/foo, when any file under this folder is changed. This is required to handle cases where specs use shared files from other folders. Specs are currently forbidden from importing files outside their top-level folder under specification, so this is a pretty good heuristic.

However, some folders are getting large enough this may be too course of a metric. An example is specification/cognitive, as shown in this run:

https://dev.azure.com/azure-sdk/internal/_build/results?buildId=3030757&view=logs&j=52599800-cfcd-5615-dd98-137d26e8cb9a&t=c0acbdc7-6ccc-5699-6c3a-c41428f73fe2&l=63

We might be able to improve Get-TypeSpec-Folders.ps1 by trying to analyze code, like the imports in *.ts files, or the settings in tsconfig.yaml, to determine exactly which specs might be impacted by changed files.

[ckairen]

closing, dup of: https://github.com/Azure/azure-rest-api-specs-pr/issues/17683