Update copilot instructions and setup steps with new stuff

[jakebailey]

The copilot-instructions.md file needs to be updated to include a few more things:

• Specify how to run a specific compiler test.
  • For submodule tests in _submodules/TypeScript: go test -run='TestSubmodule/<test name>' ./internal/testrunner.
  • For local tests in testdata/tests/cases: go test -run='TestLocal/<test name>' ./internal/testrunner.
• Indicate that issues with editor features cannot be tested with compiler tests in testdata/tests/cases.
• Indicate that this code is ported from code present in _submodules/TypeScript, and so those files should be searched for similar functions when code is either missing or potentially wrong.

Also, the setup instructions yaml needs to also:

• Run npx hereby build || true and npx hereby lint || true as well, to cache those.