feat: sort a table, sum a column, recognise a number

The formula rides the cell, as ODF writes it; Word's field syntax is what
is stored, since it is the wider language. `ooow:` in the ODF attribute
value is a namespace prefix — undeclared, LibreOffice reads it as part of
the formula and prints "Expression is faulty".

Sort takes one key and detects the type per cell where both dialogs make
you pick. Number recognition is off by default, as it is in LibreOffice,
and a formula parses a cell's text either way — exactly as there.

src/styles/editor.css

@@ -988,6 +988,12 @@
   background: var(--color-field-shade, rgba(0, 0, 0, 0.18));
 }
 
+/* A cell holding a formula shows its result on the field shade, since that result is
+   recomputed and typing over it does not last. Word draws the field the same way. */
+.tiptap :is(td, th)[data-formula] {
+  background-image: linear-gradient(var(--color-field-shade, rgba(0, 0, 0, 0.08)) 0 100%);
+}
+
 /* A citation is a field: it shows its text on the field shade both word processors use. */
 .tiptap .pm-citation {
   background: var(--color-field-shade, rgba(0, 0, 0, 0.08));