[Feature request] Stop hiding split view button and add it to the "Customize Toolbar" menu.

[menganatha]

Currently split view (aka split view editing mode aka tiling) button can only be used from the url bar and only when an already tiled tab is selected.
Problems:

1. Users have to rely on the tab context menu (opened with right click) and key shortcuts to tile their tabs. Key shortcuts (especially default) are not convenient for many users. And tab context menu is unavailable for TsT, Sideberry and similar addons users.
2. Url bar is not a very convenient place for such button for many users, it would only make sense to make it position customizable.
3. New users of Zen (like me) after watching the video from the main page (https://zen-browser.app/_astro/browser-splitview.D0FZjh2T.webm) lose their time struggling to find the said button.