Add "leave_footnote_definitions" parse option; add comrak::Node and expose NodeValue::xml_node_name.

[kivikakk]

Fixes #669.

[github-actions]

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./bench.sh ./comrak-ef57bd3	109.2 ± 9.4	105.1	150.2	1.85 ± 0.16
./bench.sh ./comrak-main	108.4 ± 1.0	106.5	110.5	1.84 ± 0.03
./bench.sh ./pulldown-cmark	59.0 ± 0.9	57.9	62.2	1.00
./bench.sh ./cmark-gfm	94.0 ± 7.8	82.3	106.7	1.59 ± 0.13
./bench.sh ./markdown-it	260.3 ± 2.0	257.5	265.6	4.41 ± 0.07

Run on Sat Nov 1 01:48:05 UTC 2025