runtime(yaml): improve syntax highlighting for YAML
- Recognize block scalar style to avoid unexpected highlighting by `yamlFlowString` (fixvim/vim#11517)
- Improve performance of `yamlFlowMappingKey` by allowing execution by the NFA engine (fixvim/vim#10730)
- It was intentionally disabled before patterns were optimized by `s:SimplifyToAssumeAllPrintable`.
- Fix detection of flow style mapping indicators (fixvim/vim#8234).
- Enable highlighting of explicit mapping value indicators and node properties in flow style.
- Add syntax highlighting tests
closes: vim/vim#14354cc7597c1ed
Co-authored-by: itchyny <itchyny@cybozu.co.jp>