Add CODEOWNERS to main (#6520)

In release branches we've added enforcement of AskMode where approval is
required before merging. This just adds the requisite file to the `main`
branch so that it will appear in future release branches. It DOES NOT
apply the branch protection policy, which is a setting controlled in
GitHub.

There is no policy change to contributions to main with this change.

.github/CODEOWNERS

@@ -0,0 +1 @@
+* @microsoft/hlsl-release