Add the ability to select the RadarChart shape (circle or polygon)

[FlorianArnould]

Use it like this :

RadarChart(
  RadarChartData(
    ...,
    radarShape: RadarShape.polygon, // nullable arg defaulting to RadarShape.circle
  ),
)

Polygon	Circle (now default / previously the only one)

[imaNNeo]

Actually everything is perfect.
I will merge it ASAP.

[codecov]

Codecov Report

Merging #1052 (87de751) into master (2de36af) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1052      +/-   ##
==========================================
+ Coverage   86.14%   86.23%   +0.08%     
==========================================
  Files          45       45              
  Lines        2758     2775      +17     
==========================================
+ Hits         2376     2393      +17     
  Misses        382      382              

Impacted Files	Coverage Δ
lib/src/chart/radar_chart/radar_chart_painter.dart	100.00% <100.00%> (ø)

📣 Codecov can now indicate which changes are the most critical in Pull Requests. Learn more

[imaNNeo]

Don't forget to update our radar_chart.md file

[FlorianArnould]

I don't know what's you process for merging the PRs, but I would like to implement this feature #883 too.
Could it be possible to merge this before to avoid the conflicts ?

[imaNNeo]

I just merged it. Thanks for contributing!