Allow to select multiple options

[phucbm]

This pull request introduces a new feature to support multiple select functionality in the EasySelect library. The changes span across multiple files to implement and integrate this feature, including updates to HTML, JavaScript, and SCSS files.

Key changes include:

HTML and JavaScript Integration:

• Added a new section in multiple-select.html to provide a UI for selecting multiple options.
• Created a new JavaScript function testMultipleSelect in test-multiple-select.js to initialize the multiple select component.
• Updated dev/script.js to include and call the new testMultipleSelect function. [1] [2]

Core Functionality Enhancements:

• Modified src/_index.js to handle multiple select logic, including methods for selecting and deselecting options and updating the UI accordingly. [1] [2] [3] [4] [5]
• Updated src/data.js to support returning selected values as an array and handle multiple selected values. [1] [2]
• Added a new module src/multi-select.js to initialize multiple select functionality.

Styling Updates:

• Enhanced styles in src/_style.scss to support the multiple select UI, including checkbox styling and layout adjustments.
• Adjusted .col-right width and alignment in dev/style.scss to accommodate the new multiple select dropdown.

Utility and Configuration Adjustments:

• Added utility functions getMultipleSelectedValues and getOptionByValue in src/utils.js to facilitate multiple select operations.
• Updated configuration constants in src/configs.js to include new classes and defaults for multiple select. [1] [2]

These changes collectively enable the EasySelect library to support multiple selection, enhancing its functionality and user experience.