Implements native sparkline support (line, column, and win/loss) in the
Xlsx reader and writer, resolving issue #4941.
- New model classes under Worksheet/Sparkline: Sparkline, SparklineGroup,
and the SparklineType enum.
- Worksheet gains a sparkline group collection with addSparkline(),
addSparklineGroup(), getSparklineGroupCollection(), and
removeSparklineGroupCollection(), plus deep-clone support.
- Reader/Xlsx/Sparklines parses x14:sparklineGroups from the sheet extLst.
- Writer/Xlsx/Worksheet emits x14:sparklineGroups; writeExtLst is
refactored so conditional-formatting data bars and sparklines share a
single extLst.
- Adds unit tests, an Xlsx round-trip test, a runnable sample, and docs.
saifulislamferoz