Emitter framework async source file

[timotheeguerin]

Extracted this part of the breaking change from #2157

With prettier 3.0.0 the formatter is async. Which requires the emitter framework to provide a way to deal with async content mutation.

This only breaks in the emitter framework is calling this.emitter.emitSourceFile directly which would happen if you wrote a custom writeOutput. If usage only include sourceFile override then it still allows returning a non promise.

-const emittedSf = this.emitter.emitSourceFile(sf);
+const emittedSf = await this.emitter.emitSourceFile(sf);

[github-actions]

Changes in this PR will be published to the following url to try(check status of TypeSpec Pull Request Try It pipeline for publish status):
Playground: https://cadlplayground.z22.web.core.windows.net/prs/2183/

Website: https://cadlwebsite.z1.web.core.windows.net/prs/2183/

[markcowl]

Breaking Change Review

Describe the breaking change
The emitter.formatTypeSpec helper in the emitter framework is now asynchronous. Callers must now await the call. Only users of the emitter framework who wrote a custom writeOuput function will be affected.

-const emittedSf = this.emitter.emitSourceFile(sf);
+const emittedSf = await this.emitter.emitSourceFile(sf);

• This was necessary to adapt to prettier 3.0
• DPG emitters do not use this API

Back-compat design to avoid immediate breaking
Any back-compat design: None

Impact
This change affecting:

• ARM
• Data-plane
• Existing specs: approx. # of service/spec impacted
• New specs
• Emitters
• SDK

Targeted Sprint
Sprint: August (release: 8/8)

Additional information
Add any other information about the change including fixes for here.

[bterlson]

I would also say that an async sourceFile is the correct design because many async things might be required to assemble your final source text. Prettier is one example but there could be many others folks would want to do (like invoke other CLI tools, make service requests, etc.)