expand abbreviations from a definition line

`*[TERM]: expansion` on its own line, PHP Markdown Extra's form, and every
whole-word use in the document becomes <abbr title="…">.

A transformer rather than an inline parser, because a definition may appear
after the use it explains and a parser only ever sees what it has already read.
A block parser rather than a pattern found later, because the line has to stop
being content — an author would notice that going wrong before anything else.

Whole-word matching is the part that would have bitten: without it a definition
of HTML quietly rewrites HTMLish and xHTML too, so both have cases. Code spans,
autolinks, raw HTML and an already-expanded term are skipped, the longest
definition wins where two could match, and the expansion is escaped into the
attribute so a quoted phrase cannot end it.

Definitions are document-scoped. A term defined in a page does not reach an
included fragment, which is parsed on its own bytes exactly as footnotes are —
stated in content-model.md rather than left to be discovered.

The nesting gate caught firstMatch four levels deep; the inner search is its own
function now, which reads better than it did before the warning.

core 2794/2800, ext 1483/2000, 34 gates green, 0 warnings.

examples/demo-site/content/writing/notes-on-water/index.en.md

@@ -19,7 +19,9 @@ The page has a footnote of its own[^page], a table, a definition list and ~~a st
 The notation marks cover what prose needs most often: the gauge measures H~2~O to about 10^-3^ m, and the
 ==important== column is the second. Where there is no mark for something, HTML does it: the reading is taken
 with <kbd>Shift</kbd> held. Content in the site root is the author's own, so it renders rather than being
-dropped.
+dropped. Readings go to the NIWA archive, and the definition below expands every use of it.
+
+*[NIWA]: National Institute of Water and Atmospheric Research
 
 | Gauge | Reading | Note |
 |-------|---------|------|