Add load theme function to load themes with window context

[umairabid]

Enhancement: Dynamic ECharts Theme Loading

This pull request introduces a new function, RailsCharts.loadTheme, to improve how ECharts themes are managed and loaded within the gem. When loaded via rails the IIFE of themes seems to miss the context of windows and hence this becomes undefined. This PR fixes it by adding a function to dynamically load the themes file, while using windows as context

How to Use

Users should now import themes by calling the new RailsCharts.loadTheme function

// application.js
import "echarts" 
import "echarts.themeloader"

// Load the desired theme dynamically
RailsCharts.loadTheme('dark');

[umairabid]

Fixes #34 and closes #25

[igorkasyanchuk]

@umairabid could you please add this to readme. And I think this should be mentioned as alternative option how to load themes doesn't work.

[umairabid]

@igorkasyanchuk Updated

[igorkasyanchuk]

thank you! plan to release today