✨ NEW: Add highlight_verbatim option to skip the <pre><code> wrapper (#256)

[cjchanh]

Add highlight_verbatim option to skip the <pre><code> wrapper

Fixes #256.

Summary

New highlight_verbatim option: when set, the renderer passes the
highlighter's output through byte-exact — no <pre><code> wrapper, no
language-class injection, no renderer-added newline. The highlighter's return
value is the final rendered block, byte for byte.

Motivation (from #256)

When highlight returns pygments-style output (starts with <div, not
<pre>), markdown-it-py wraps it in <pre><code>…</code></pre> anyway,
forcing users to rewrite their Pygments CSS themes for the duplicated
wrapper. Existing workarounds require subclassing RendererHTML and
reimplementing fence().

Behavior

• highlight_verbatim=False (default): behavior byte-identical to today
  for every existing case, including the <pre-continues heuristic.
• highlight_verbatim=True: fence() returns the highlighter's output
  untouched. The option is inert without a highlight callback.

Byte-exactness guarantee

The renderer does not append, strip, or alter a single byte — trailing
newline or none, the output equals the highlighter's return. Regression-tested
in both directions.

Tests

Nine tests in tests/test_api/test_highlight_verbatim.py: default wrapping
unchanged, <pre passthrough, non-<pre passthrough, byte-exactness both
ways, language-class injection skipped, instance-toggle, default-path
inertness. Full suite: 991 passed.

Verification

pip install -e . && pytest tests/

Docs & changelog

docs/using.md updated; CHANGELOG.md entry added.