Added CODEOWNERS file (#5544)

CODEOWNERS

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+# CODEOWNERS file for Actual Budget
+# Please add your name to code-paths that you feel especially 
+# passionate about. You will be notified for any PRs there.
+
+/packages/api/                                    @MatissJanis
+/packages/component-library/                      @MatissJanis
+/packages/desktop-client/src/components/mobile    @joel-jeremy 
+/packages/desktop-electron/                       @MikesGlitch
+/packages/loot-core/src/server/budget             @youngcw
+/packages/sync-server/                            @matt-fidd

upcoming-release-notes/5544.md

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+---
+category: Maintenance
+authors: [MatissJanis]
+---
+
+Add CODEOWNERS file to assign ownership for specific directories in the repository.
+