Add Pareto option for Highcharts

Problem/Motivation

Pareto charts (80/20 rule) are essential business intelligence tools used to identify the most significant factors in a data set. While Highcharts supports Pareto functionality via a dedicated module, the Drupal Charts Highcharts submodule currently lacks an interface to enable this feature. Adding this allows users to overlay a cumulative percentage line on bar and column charts without manual #raw_options configuration.

Steps to reproduce

N/A (Feature Request)

Proposed resolution

• Register the Highcharts pareto.js library in charts_highcharts.libraries.yml.
• Add a global configuration toggle to enable/disable the Pareto library across the site.
• Enhance the Bar and Column chart configuration forms to include an "Enable Pareto line" checkbox and a "Pareto line color" picker.
• Automate the generation of the secondary Y-axis (Percentage) and the linked Pareto series in the Highcharts plugin logic.
• Ensure cumulative values are formatted to two decimal places for tooltips, data labels, and axis labels for better readability.

Remaining tasks

• Review the patch.
• Test with various data sets to ensure the secondary axis scaling is correct.

User interface changes

• Global Settings: A new checkbox "Enable Highcharts' Pareto library" in the Highcharts module configuration.
• Chart Settings: When "Bar" or "Column" is selected, new fields for "Enable Pareto line" and "Pareto line color" will appear in the library-specific options section.

API changes

None. This utilizes existing Highcharts plugin hooks.

Data model changes

The charts_highcharts.schema.yml is updated to include pareto_library in global settings and pareto_line/pareto_color in the bar/column mapping.