Convert <pre> to Markdown ``` and vice-versa with nonBreakableLines

The HTML parser calls QTextBlockFormat::setNonBreakableLines(true) when
it sees a <pre> tag; so for symmetry, the markdown reader now does the
same when it sees a fenced code block, and the markdown writer honors
the nonBreakableLines property by writing a fenced code block. This
preserves the meaning better when reading HTML and writing markdown or
vice-versa, without modifying HTML reading or writing code.

Added a test tst_QTextMarkdownImporter::fencedCodeBlocks() which
unfortunately also highlights a known bug in the markdown reader: each
fenced code block ends with an extra empty block. That can be fixed
separately.

tst_QTextMarkdownWriter::fromHtml(preformats with embedded backticks)
that we re-enabled in 1abaf9d5d6ea9c6554362e851903ddd214a6f659 was not a
very useful test: ``` with a space and some words but no newline is not
a fence: it's just like a `monospace` span. We have had trouble with
those in CI because of missing monospace fonts, or inconsistency when
a supposedly mono font's QFontInfo::fixedPitch() returns false.
So just test proper <pre>/fence conversion for now.

Pick-to: 6.3
Fixes: QTBUG-100515
Fixes: QTBUG-100981
Task-number: QTBUG-101031
Change-Id: I88f0ede0810d8a9480b30eb0cd780e1af67cc5f2
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

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