MarkdownWriter: append unescaped spans in bulk - #456

During profiling after the release of Tika 4.0.0, we found we could improve write speed quite a bit in cases where there weren't a lot of characters that needed to be escaped.

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text() always passes a CharMatcher, so every text literal was written 
to the Appendable one character at a time. Scan for the next character 
that needs escaping and append the clean span in one call instead.

JMH, whole CommonMark spec (SpecBenchmark, JDK 17, -f 3 -wi 8 -i 12)

Rendering 4.7 MB of plain prose (best-of-22 after warmup, JDK 17)

Same measurement, 4.0 MB of the spec (escape-dense, closer to a worst case)