Fix mathjax wrap newline handling and add regression test

[Arthur-Milchior]

If you start by writing the mathjax and then wrapping it with the mathjax tools, the new lines are incorrectly rendered as <br>. This problem exists generally with any formatting, but in my experience, the new line case is the most common one. In particular because there is currently zero way to add the new line properly before wrapping the math in mathjax.

Further correction will wait until someone with decision power give feedback on the issue request.

This change is done with a simple search and replace and not parsing HTML, for the sake of the simplicity, and to ensure this still works if HTML is somehow badly formatted.

Refs #5106

[Arthur-Milchior]

  1) ts/tests/e2e/context-switching.spec.ts:171:5 › chooser UI and session behaviour › notetype chooser updates field list via the Svelte path 

    TimeoutError: page.waitForSelector: Timeout 15000ms exceeded.
    Call log:
      - waiting for locator('.note-editor') to be visible


       at fixtures.ts:37

      35 |         await installBridgeStub(page);
      36 |         await page.goto("/editor/?mode=add", { waitUntil: "domcontentloaded" });
    > 37 |         await page.waitForSelector(".note-editor", { timeout: 15_000 });
         |                    ^
      38 |         await use(page);
      39 |     },
      40 |
        at Object.editorPage (/home/runner/work/anki/anki/ts/tests/e2e/fixtures.ts:37:20)

As far as I can see, the current CI error in unrelated to this PR