reader: wide tables scroll sideways from the keys

`zl` / `zh` scroll the table, code block or formula at the top of the
view a few columns right or left, and `zL` / `zH` by half its width, as
vim scrolls a long line. It is the first one too wide for the column that
reaches down past the top of the reading view, so it is the one being
read. A CSV's table in the file pane answers the same keys. Before, those
only scrolled with the pointer or a trackpad.

web/src/lib/Help.svelte

@@ -16,6 +16,7 @@
     ['j / k · ↓ ↑', 'move down / up the list — scroll the document when it has focus'],
     ['gg / G', 'first / last row, or top / bottom of the document — a first g, d, ] or [ waits a second in the status line'],
     ['Ctrl-D / Ctrl-U', 'half a page down / up the open document'],
+    ['zl / zh · zL / zH', 'the wide table or code block at the top of the view, sideways — a little, or half its width'],
     [']] / [[', 'next / previous heading of the document — in the editor too'],
     ['gO', 'the headings as a list: j/k, Enter jumps there'],
     ['F', 'hints over the links and buttons in view — type one to follow it'],