feat(utils): support plugins with configuration objects

[Hugoer]

Description

This PR fixes the way PostCSS plugins are loaded by adding support for plugin entries defined as [pluginName, options].

Previously, only string-based plugin names were allowed, which prevented the use of more advanced plugins that require configuration objects (e.g. postcss-extract-media-query). This change ensures that both string and array formats are supported when defining PostCSS plugins in the build configuration.

Unit tests have been added to verify proper behavior, including:

• Loading plugins as strings
• Loading plugins with options
• Skipping invalid or falsy entries
• Throwing on malformed plugin definitions

This update improves the flexibility and compatibility of the build pipeline with the wider PostCSS ecosystem.

Related Issue

Fixes #123

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• I have read the CONTRIBUTING document.
• All new and existing tests passed.