digraphs: reclassify vim support levels

Instead of custom u-* classes, reuse common l1-5 in include:

.u-di      → l4 (vim + rfc)
.u-prop    → l3 (vim)
.u-prop.ex → l2 (shiar)
.ex        → l1 (rfc only)

Prepend u- on page to avoid conflicting with script styling (leave
background colour unaltered).

Loses ascii/latin1 distinction, but clarifies (vim) compatibility and
allows for easier extension later on.