[reggen, regtool] Request for IEEE1685-2022/Accellera IPXACT XML Schema Generation Support in `regtool`

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### Description

Dear Maintainers,

I am writing to request the addition of support for generating IEEE/Accellera IPXACT XML Schema in `regtool`.

**Background:**

IPXACT is an XML-based standard (IEEE 1685-2022) for packaging, integrating, and reusing IP within tool flows. It provides a common language for representing IP metadata, enabling interoperability between different tools and platforms.

**Benefits of IPXACT Support in `regtool`:**

* **Improved Interoperability:** Generating IPXACT XML Schemas in `regtool` would allow users to easily exchange IP metadata with other tools that support IPXACT.
* **Enhanced Reusability:** IPXACT schemas can be used to define reusable IP components, facilitating the creation and maintenance of complex IP designs.
* **Standardization:** Adhering to the IEEE/Accellera IPXACT standard would ensure that `regtool`-generated schemas are compatible with industry best practices.
* **Leveraging Existing Capabilities:** As `regtool` already has the capability to parse and generate various formats, adding support for IPXACT would be a relatively straightforward extension.
* **Simplified Integration:** By supporting IPXACT, `regtool` would become a more comprehensive tool for IP management, simplifying the integration of IP into different tool flows.

**Current Limitations:**

While `regtool` currently supports generating various formats, it does not include support for IPXACT XML Schemas. This limits the interoperability and reusability of `regtool`-generated IP metadata.

**Request:**

I kindly request that the `regtool` team consider adding support for generating IEEE/Accellera IPXACT XML Schemas. This feature would greatly enhance the utility of `regtool` for IP design and management.

**Additional Information/References:**

https://www.accellera.org/downloads/standards/ip-xact
https://ieeexplore.ieee.org/document/10054520
https://www.accellera.org/images/downloads/standards/ip-xact/1685-2022-VE-1.0-Specification.zip

Thank you for your time and consideration. I look forward to hearing from you soon.

Start with regtool's existing format-generation capabilities and compare them with the IEEE/Accellera IPXACT references linked in the issue. Define the required generation scope and compatibility criteria; done means regtool can generate valid IEEE 1685-2022 IPXACT XML Schemas.

Written by the indexing model from the issue text.