feat: enhanced line chart curve function

[huanghui1998hhh]

This PR enhances the curve API:

1. Keeps the existing curve algorithm.
2. Add a new curve algorithm copied from ECharts.
3. Allows users to provide their own curve drawing logic in whatever way they prefer.
4. Fully preserved the existing implicit animation when toggling between curved and straight lines.
5. The old API has not been removed. It is marked as deprecated and remains backward-compatible.

Before (old params)	After (new param)
isCurved: false	curve: LineChartCurve.noCurve
isCurved: true	curve: const LineChartCubicTensionCurve()
isCurved: true, curveSmoothness: S, preventCurveOverShooting: P, preventCurveOvershootingThreshold: T	curve: const LineChartCubicTensionCurve(smoothness: S, preventCurveOverShooting: P, preventCurveOvershootingThreshold: T)

I’m not entirely sure whether these decisions align with your original vision, please give me some advice. I'll complete the follow-up work.
@imaNNeo

[codecov]

Codecov Report

❌ Patch coverage is 99.31973% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.91%. Comparing base (32e75f5) to head (47e5dbc).

Files with missing lines	Patch %	Lines
lib/src/chart/line_chart/line_chart_curve.dart	99.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2000      +/-   ##
==========================================
+ Coverage   92.49%   92.91%   +0.42%     
==========================================
  Files          50       51       +1     
  Lines        3731     3855     +124     
==========================================
+ Hits         3451     3582     +131     
+ Misses        280      273       -7     

Flag	Coverage Δ
flutter	92.91% <99.31%> (+0.42%)	⬆️

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[imaNNeo]

It looks good!
Can you please write the needed unit-tests and update the documentation? both in our dart code and markdown files.

[huanghui1998hhh]

@imaNNeo Done! Please review.