[CLI] Added a "\n" to end of writefile (#1251)

## Summary
Added '\n' to end of `WriteFile()` in `cuecfg` package. This resolves an
issue of devbox.json unnecessarily showing up in PR diffs because the
newline at the end of the file is removed by CLI and was added by IDE
formatters.

However, this function is used to write the config to a file as well as
a couple of other cases such as in `auth` package.
I'm guessing the `\n` for cases other than writing config won't cause an
issue. But I haven't tested those cases.

Addresses #1248 

## How was it tested?
- compile
- `./devbox init` has a newline at the end of devbox.json and
devbox.lock
- `./devbox add hello` keeps the newline at the end of devbox.json and
devbox.lock

Author: mohsenari

internal/cuecfg/cuecfg.go

@@ -82,7 +82,7 @@ func WriteFile(path string, value any) error {
 	if err != nil {
 		return errors.WithStack(err)
 	}
-
+	data = append(data, '\n')
 	return errors.WithStack(os.WriteFile(path, data, 0644))
 }