Add `modeSuffix` to default sources template

> ```
> sourcesTemplate: '{{name}}-{{version}}-sources{{modeSuffix}}.zip'
> artifactTemplate: '{{name}}-{{version}}-{{browser}}{{modeSuffix}}.zip'
> ```

Yes, we should do this in the next major version.

_Originally posted by @aklinker1 in https://github.com/wxt-dev/wxt/issues/1605#issuecomment-2831768920_
            