[Frame] Merge search params instead of overwriting them

[titouanmathis]

When using the Frame component with a FrameForm, when the FrameForm form is submitted, the URL used to fetch the new content overwrite any predefined search params here: https://github.com/studiometa/ui/blob/develop/packages/ui/organisms/Frame/Frame.ts#L145

It might be useful to merge the generated search params from the form data with any existing one.

for (const [param, value] of new URLSearchParams(new FormData(target.$el))) {
  url.searchParams.set(param, value);
}