fix: require whitespace after an ATX heading marker

[RobinDev]

Closes part of #37 (point 4).

HeadingRule::shouldParse() only checked that the line starts with #, so any #-prefixed word became a heading:

new Markdown(highlighter: null)->parse('#titre')->html;   // <h1 id="titre">titre</h1>
new Markdown(highlighter: null)->parse('#hashtag')->html; // <h1 id="hashtag">hashtag</h1>

CommonMark §4.2: an ATX heading is "a string of characters, parsed as inline content, between an opening sequence of 1–6 unescaped # characters and an optional closing sequence", and "the opening sequence of # characters must be followed by spaces or tabs, or by the end of line".

shouldParse() now checks both conditions:

Input	Before	After
#titre	<h1 id="titre">titre</h1>	<p>#titre</p>
#5 bolt	<h1 id="5-bolt">5 bolt</h1>	<p>#5 bolt</p>
####### seven	<h7>	<p>####### seven</p>
# Titre	<h1 id="titre">Titre</h1>	unchanged
#	<h1></h1>	unchanged

The seven-marker case also stops emitting an <h7> element, which isn't HTML.

composer test (331 tests), lint and analyse are clean.

[github-actions]

Benchmark Results

Comparison of fix/atx-heading-space against main (8449b00fcd8912b6050f53baa7795e8cdf45ac70).

Open to see the benchmark results

No benchmark changes above ±5%.

_{Generated by phpbench against commit 8e924a0}

[brendt]

Please fix the merge conflicts before merging

[RobinDev]

Rebased on main, conflicts resolved — the overlap was with the generateIds constructor from #46 and the extra test cases, both kept.

While re-checking it against a personal dataset of real-world Markdown, this turns out to change a few hundred documents, all in the same direction. A line starting with #[ — a link at the very start of a line — is currently swallowed as a heading:

#[Book now](#reservation)

main	<h1 id="reservation)"><a href="">Book now</a></h1>
this PR	<p>#<a href="#reservation">Book now</a></p>

Note the href="": the heading rule consumes the whole line before the link rule sees it, so the destination is lost and the remainder of the line ends up inside the id attribute. The second column is what league/commonmark produces.