runtime(yaml): improve syntax highlighting for YAML

- Recognize block scalar style to avoid unexpected highlighting by `yamlFlowString` (fix #11517)
- Improve performance of `yamlFlowMappingKey` by allowing execution by the NFA engine (fix #10730)
  - It was intentionally disabled before patterns were optimized by `s:SimplifyToAssumeAllPrintable`.
- Fix detection of flow style mapping indicators (fix #8234).
- Enable highlighting of explicit mapping value indicators and node properties in flow style.
- Add syntax highlighting tests

closes: #14354

Signed-off-by: itchyny <itchyny@cybozu.co.jp>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/syntax/testdir/dumps/yaml_05.dump b/runtime/syntax/testdir/dumps/yaml_05.dump
new file mode 100644
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