Function to take flash message without updating the flash message store

[Stadly]

takeFlash can be used to prevent the flash message store from being updated with the flash message. This can be useful for applying custom handling to the flash message in progressively enhanced forms:

<script lang="ts">
  import { toast } from "svelte-sonner";
  import { takeFlash } from 'sveltekit-flash-message';
  import { superForm } from "sveltekit-superforms";

  let toastId: string | number | null = null;
  const { form, enhance } = superForm(
    { test: "" },
    {
      onSubmit: () => {
        toastId = toast.loading("Pending...");
      },
      onResult: async ({ result }) => {
        if (toastId !== null) {
          // Prevent the flash message store in `src/routes/+layout.svelte` from being updated.
          // Instead, we manually take the flash message here and update the already pending toast.
          const flash = await takeFlash();
          if (!flash) return;
          if (result.type === "error" || result.type === "failure") {
            toast.error(flash.message, {
              id: toastId,
              action: { label: "Retry", onClick: () => { /* Retry logic here */ }, },
            });
          } else {
            toast.success(flash.message, { id: toastId });
          }
        }
      },
    },
  );
</script>

<form method="POST" action="/test" use:enhance>
  <input type="text" name="test" bind:value={$form.test} />
  <button>Submit</button>
</form>

[Stadly]

On second thought, I think this is probably better done in userland by having more logic around the flash store subscription in +layout.svelte. Feel free to reopen if you disagrees and think this PR is worth merging.