Add support for Classic pipeline mode

[warrenbuckley]

Related to #654

Adds support for Classic pipeline mode in IIS Express integration for VS Code, enhancing flexibility for project configurations.

• Updates IISExpress.ts:
  • Introduces a new optional property pipelineMode in the IExpressArguments interface to allow specifying the pipeline mode.
  • Modifies the application pool assignment logic to support both Integrated and Classic pipeline modes based on the CLR version and the newly introduced pipelineMode property.
• Modifies iisexpress-schema.json:
  • Extends the JSON schema with a new pipelineMode property, allowing users to specify their desired pipeline mode in the iisexpress.json configuration file.
• Adjusts settings.ts:
  • Adds handling for the pipelineMode property within the getSettings function, ensuring the new configuration option is properly processed and applied.

These changes address the need for Classic pipeline mode support, as requested in the issue, by providing the necessary options and logic to configure the pipeline mode for IIS Express directly from the VS Code extension settings.

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