Include @dotnet/source-build-internal as code owners for SB files

[ellahathaway]

Summary

@dotnet/source-build-internal is not tagged as a code owner for SourceBuildPrebuiltBaseline.xml. It would be helpful to create a CODEOWNERS file and include source-build-internal on SB files. For reference, Aspnet does this in their CODEOWNERS file: https://github.com/dotnet/aspnetcore/blob/36c976723fd0b55dad80f1ff32e665800ea4a021/.github/CODEOWNERS#L19

Background and Motivation

Having a code owners file would also be generally useful when opening PRs in this repo, as it can sometimes be difficult and time consuming to determine who should be included as a reviewer.

Proposed Feature

Create a CODEOWNERS file

Alternative Designs

No response

[ellahathaway]

Can this issue be closed?

[rainersigwald]

It'll auto-close when #9532 brings the change to main.