pub(crate) fn set_ctx(self, cx: Scope) -> &Self {
        provide_context(cx, self)
    }
Places this instance of `RenderCtx` into Sycamore's context system,
returning a reference. This assumes no other instances of `RenderCtx`
have been added to context already (or Sycamore will cause a panic).
Once this is done, the render context can be modified safely with
interior mutability.
packages/perseus/src/template/render_ctx.rs