[Forwardport] Allows modules with underscores in name to set custom a frontend_model in system.xml

[mage2pratik]

Original Pull Request

#14397

This issue is the same as this issue except that it applies to the system.xml file rather than layout XML files.

Description

Currently it's not possible to set a custom frontend_model value in system.xml if the module you're using has an underscore in the module name. This limit is put in place via a regular expression defined in the app/code/Magento/Config/etc/system_file.xsd file.

The supplied fix simply adds the underscore character to the regular expression.

Manual testing scenarios

To test simply create a configuration option that uses a custom frontend_model.

[magento-engcom-team]

Hi @mage2pratik. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
• @magento-engcom-team give me {$VERSION} instance - deploy vanilla Magento instance

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[magento-engcom-team]

Hi @sidolov, thank you for the review.
ENGCOM-2712 has been created to process this Pull Request

[magento-engcom-team]

Hi @mage2pratik. Thank you for your contribution.
We will aim to release these changes as part of 2.3.0.
Please check the release notes for final confirmation.