fix: apply GFM flanking rules to strikethrough delimiters

[RobinDev]

A tilde delimiter opens and closes strikethrough regardless of what surrounds it, so text that merely uses ~ as "approximately" gets struck:

~11km / ~750m de dénivelé

main	<p><s>11km / </s>750m de dénivelé</p>
this PR	<p>~11km / ~750m de dénivelé</p>

The GFM strikethrough extension uses the same flanking rules as emphasis: an opening run may not be followed by whitespace, a closing run may not be preceded by it, and the two runs must be the same length. league/commonmark leaves every case in the table below alone; main strikes four of them.

shouldParse() is unchanged. parse() now looks for a run that can actually close, and falls back to the literal text when there is none:

Markdown	main	this PR	league/commonmark
~struck~	<s>	<s>	<del>
~~struck~~	<s>	<s>	<del>
a~struck~b	<s>	<s>	<del>
~ struck ~	<s>	literal	literal
~11km / ~750m	<s>11km / </s>750m	literal	literal
~struck~~	<s>	literal	literal
~~struck~	<s>	literal	literal

Worth fixing now because #47 widened it. Those rule sets did not run StrikethroughRule before, so a blockquote, a list item or a table cell could not hit this; they can now. On a personal dataset the count of wrongly struck documents roughly tripled when #47 landed, and drops to zero with this PR.

A run of three or more tildes no longer delimits strikethrough either, which is what league does and what the fenced-code rule from #42 already assumes.

[github-actions]

Benchmark Results

Comparison of fix/strikethrough-flanking against main (b4cd28e8892eb3049af2bc67612f34149ada5aa7).

Open to see the benchmark results

No benchmark changes above ±5%.

_{Generated by phpbench against commit 60cd004}