Fix widget-e2e: add null-loader for CSS files to fix maplibre-gl compilation

[imatiach-msft]

Description

This pull request updates the Cypress plugins configuration for the widget end-to-end tests to improve compatibility and streamline test execution. The most significant changes involve customizing the webpack configuration to handle CSS imports and ensure compatibility with Node 20.

Webpack configuration improvements:

• Added a custom webpack configuration (customizeWebpackConfig) to ignore CSS file imports by using null-loader, preventing errors from dependencies that require CSS during e2e tests.
• Disabled webpack caching in the custom config to ensure fresh transpilation and compatibility with Node 20.

Cypress preprocessor update:

• Updated the TypeScript preprocessor setup to use the new custom webpack configuration, ensuring the above changes are applied during test runs.

Checklist

• I have added screenshots above for all UI changes.
• I have added e2e tests for all UI changes.
• Documentation was updated if it was needed.

[Copilot]

Pull request overview

This pull request brings the widget-e2e Cypress configuration in line with dashboard-e2e by adding webpack customization to handle CSS imports from dependencies like maplibre-gl (a transitive dependency through plotly.js). The changes ensure e2e tests can compile successfully in Node 20 environments.

Changes:

• Added custom webpack configuration to ignore CSS file imports using null-loader
• Disabled webpack caching for Node 20 compatibility
• Updated TypeScript preprocessor to use the custom webpack configuration

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