Add code owners for /shared/

Author: owen-mc

CODEOWNERS

@@ -9,6 +9,7 @@
 /python/ @github/codeql-python
 /ruby/ @github/codeql-ruby
 /rust/ @github/codeql-rust
+/shared/ @aschackmull @hvitved @owen-mc
 /swift/ @github/codeql-swift
 /misc/codegen/ @github/codeql-swift
 /java/kotlin-extractor/ @github/codeql-kotlin