update .gitignore for dev output (#10)

### TL;DR

Added `packages/backend/dev/` to `.gitignore` file.

### What changed?

`packages/backend/dev/` is now ignored from the git repository for better development practices.

### How to test?

Try adding files to `packages/backend/dev/`, they should not be tracked by git now.

### Why make this change?

To ensure unnecessary development files don’t accidentally get checked into the repository.

.gitignore

@@ -5,4 +5,5 @@ yarn-error.log
 .yarnrc
 
 packages/backend/environments/local.toml
+packages/backend/dev/
 packages/frontend/dist/