feat: Customizable essentials tab amount per row

[ErdemGKSL]

I basically added a customization for styling the number of essentials per row.

Note: Tested for collapsed toolbar too, it doesn't get broken but also doesn't work when collapsed toolbar. It works fine, i will add image of it.

Localization added (l10n/en-US/browser/browser/preferences/zen-preferences.ftl): zen-browser/l10n-packs#161

Images:

[blakegearin]

Wanted to note I'm going to pull this down and make sure it plays nicely with #8441

[mr-cheffy]

Thank you for your contribution and the time you took to submit this PR regarding configurable essential tabs per row.

That said, we’ve decided not to merge this change at this time.

Our layout system is deliberately structured and tightly tuned to ensure a consistent and optimal user experience across a wide range of screen sizes and use cases. Introducing configurability in this area could introduce edge cases and break the visual balance we’ve worked hard to achieve.

We also want to note that for users or developers who need more customization, the system is already mod-friendly, and adjustments like this can be implemented externally through mods without affecting the core layout logic.

Again, thank you for your contribution and understanding. We welcome further suggestions and PRs, especially those aligned with the design direction and maintainability of the core project.