Fix invalid CSS handling in HtmlWidget

[daohoangson]

Related to #1287

Address the issue of invalid CSS causing errors in HtmlWidget.

• Add isValidCss function: Add a new function isValidCss in packages/core/lib/src/core_helpers.dart to check if a CSS string is valid.
• Update HtmlWidget: Modify HtmlWidget in packages/enhanced/lib/src/html_widget.dart to ignore invalid CSS errors in the onErrorBuilder callback.
• Add unit tests: Add unit tests for the isValidCss function in packages/core/test/src/core_helpers_test.dart.
• Update documentation: Update packages/enhanced/README.md to include examples on handling invalid HTML/CSS in HtmlWidget.

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