Refactors open_new_blocks by lifting out handlers

[digitalmoksha]

Another round of refactoring of the open_new_blocks function, lifting out the handlers.

[github-actions]

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./bench.sh ./comrak-0402355	314.7 ± 3.6	311.8	330.5	2.83 ± 0.04
./bench.sh ./comrak-main	318.3 ± 3.8	315.7	335.7	2.87 ± 0.04
./bench.sh ./pulldown-cmark	111.1 ± 0.6	110.0	112.4	1.00
./bench.sh ./cmark-gfm	119.0 ± 0.8	118.1	120.7	1.07 ± 0.01
./bench.sh ./markdown-it	471.9 ± 1.5	469.3	476.0	4.25 ± 0.03

Run on Fri Dec 13 20:42:00 UTC 2024

[digitalmoksha]

I'm going to go ahead and optimistically merge this.