fix: declare java.xml module dependency for Java 9+ modular usage

[JinlongLiao]

What this PR does / why we need it?

This PR fixes the IllegalAccessError that occurs when using fastjson2 (v2.0.61) in Java 9+ modular environments. The error is caused by missing java.xml module declaration in src/main/moditect/module-info.java, while the code internally references org.w3c.dom.Node from java.xml.

Without this fix, users cannot properly use fastjson2 in modularized projects, as the JVM prevents access to java.xml classes.

Summary of your change

• Updated src/main/moditect/module-info.java to explicitly declare requires java.xml;.
• Ensured that ObjectWriterProvider and related XML handling code can access org.w3c.dom.Node without IllegalAccessError.
• Verified compatibility with Java 9+ modular projects.
• Added regression tests to confirm XML serialization works under modularized environments.

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

[CLAassistant]

All committers have signed the CLA.

[wenshao]

No issues found. LGTM! ✅ — gpt-5.5 via Qwen Code /review