Update footnote regex to handle footnotes with spaces — radicle-explorer

Also updates the markdown fixture to test footnotes with spaces

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   renderer: (token: Tokens.Generic): string =>
     `<sup class="txt-tiny footnote-ref" id="${referencePrefix}:${token.text}"><a href="#${footnotePrefix}:${token.text}">[${token.text}]</a></sup>`,
 };
 const footnoteMatch = /^\[\^([^\]]+)\]:\s([\S]*)/;
 const footnoteMatch = /^\[\^([^\]]+)\]:\s([\S].*)/;
 const footnoteMarkedExtension = {
   name: "footnote",
   level: "block",

   });
   await expect(
     page.locator(
       "text=This is an example footnote[0]. And some radicle[1] examples.",
       "text=This is an example footnote[1]. And some radicle[2] examples.[3]",
     ),
   ).toBeVisible();
   await page.getByRole("button", { name: "Collapse" }).click();
   await expect(page.getByText("0. https://example.com ↩")).toBeVisible();
   await expect(page.getByText("1. https://radicle.xyz ↩")).toBeVisible();
   await expect(page.getByText("1. https://example.com ↩")).toBeVisible();
   await expect(page.getByText("2. https://radicle.xyz ↩")).toBeVisible();
   await expect(
     page.getByText(
       "3. A corporeal grounding, though one hardly to the exclusion of the cerebral as in the erroneous sense of a mind/body dichotomy, except insofar as what is most squarely in the cerebral would land here only on the periphery of our focus. ↩",
     ),
   ).toBeVisible();
   await expect(page).toHaveScreenshot({ fullPage: true });
   await page.getByText("Code").click();
   await expect(
     page.locator(
       "text=This is an example footnote[^0]. And some radicle[^1] examples.",
       "text=This is an example footnote[^1]. And some radicle[^2] examples.[^3]",
     ),
   ).toBeVisible();
   await expect(page.getByText("[^1]: https://example.com")).toBeVisible();
   await expect(page.getByText("[^2]: https://radicle.xyz")).toBeVisible();
   await expect(
     page.getByText(
       "[^3]: A corporeal grounding, though one hardly to the exclusion of the _cerebral_ as in the erroneous sense of a mind/body dichotomy, except insofar as what is most squarely in the cerebral would land here only on the periphery of our focus.",
     ),
   ).toBeVisible();
   await expect(page.getByText("[^0]: https://example.com")).toBeVisible();
   await expect(page.getByText("[^1]: https://radicle.xyz")).toBeVisible();
 });
 test("math", async ({ page }) => {