Added gradient to FlLine

[Dartek12]

Adds a gradient option to the FlLine.

Example usage of gradient for horizontal line:

Issue: #1197

[imaNNeo]

Everything seems great! Thanks for contributing

There is just one thing to do, can you please update the current documentation in the base_chart.md?

After that, we're ready to merge!

[codecov]

Codecov Report

Merging #1446 (95c18f4) into master (79e6ec7) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1446      +/-   ##
==========================================
+ Coverage   86.54%   86.62%   +0.08%     
==========================================
  Files          45       45              
  Lines        2996     3014      +18     
==========================================
+ Hits         2593     2611      +18     
  Misses        403      403              

Files	Coverage Δ
.../src/chart/base/axis_chart/axis_chart_painter.dart	73.75% <100.00%> (+0.98%)	⬆️
lib/src/chart/line_chart/line_chart_painter.dart	92.05% <100.00%> (+0.10%)	⬆️
lib/src/extensions/paint_extension.dart	100.00% <100.00%> (ø)

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[Dartek12]

I have updated the documentation as requested

[imaNNeo]

Can you please rebase your branch?

[Dartek12]

@imaNNeo rebased as requested

[imaNNeo]

Thanks for contributing!