Add copilot instructions to help emitters upgrade TCGC (#8940)

This PR introduces instructions for copilot to help with TCGC upgrades.
These instructions can be activated by prompting copilot in agent mode
the following:

`tcgc upgrade <emitter-name> <new-tcgc-version>`

This will activate the instructions with which copilot will resolve the
emitter to work on and systematically upgrade the package, this is its
workflow:

1. **Pre-migration assessment** - Detect build system and run initial
build
2. **Package upgrade** - Update package.json and install dependencies
3. **Post-upgrade build** - Run build to identify breaking changes
4. **Fix breaking changes** - Apply code changes to resolve issues
5. **Final validation** - Run build to ensure all issues resolved
6. **Summary and documentation** - Report changes to user

Notes:

* Copilot is instructed to query npm before attempting to edit
`package.json` to make sure the versions to be added actually exist.
* Copilot is instructed to query for TCGC peer dependencies and
understand the dependency graph
* Copilot is instructed to iterate until all the peer dependency
conditions are met, including for packages not directly related to TCGC
(i.e. @azure-tools/azure-http-specs)

Author: joheredi

.github/copilot-instructions.md

@@ -120,3 +120,4 @@ TypeSpec is a language for defining cloud service APIs and shapes. This monorepo
 - [Testserver Generation](./prompts/testserver-generation.md): Instructions for generating TypeSpec HTTP spec test servers
 - [http-client-csharp Development](./instructions/http-client-csharp.instructions.md): Instructions for developing the C# HTTP client
 - [http-client-java Development](./prompts/http-client-java-development.md): Instructions for developing the TypeSpec library for Java client.
+- [TCGC Upgrade](./prompts/upgrade-tcgc.instructions.md): Instructions for TCGC version on emitters. Activate with: `tcgc upgrade <emitter-name> <new-version>`