Bump version to 0.64.0

[imaNNeo]

• BUGFIX (by @Anas35) Fix Tooltip not displaying when value from BackgroundBarChartRodData is less than zero. Tooltip not displaying when touched value from BackgroundBarChartRodData is less than zero. #1345.
• BUGFIX (by @imaNNeo) Fix Negative BarChartRodStackItem are not drawn correctly bug, Negative BarChartRodStackItem are not drawn correctly in Flutter 3.10.5 (stable) #1347
• BUGFIX (by @imaNNeo) Fix bar_chart_helper minY calculation bug, BarChart isn't fit to its visible area #1388
• IMPROVEMENT (by @imaNNeo) Consider fraction digits when formatting chart side titles, problems with tick step values on X axis #1267

[codecov]

Codecov Report

Merging #1455 (633c40d) into master (621b79e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1455   +/-   ##
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  Coverage   86.54%   86.54%           
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  Files          45       45           
  Lines        2996     2996           
=======================================
  Hits         2593     2593           
  Misses        403      403           

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