Fix readonly serializing on structs/string

[IXLLEGACYIXL]

PR Details

Fixes Serialization of readonly members

Description

1. readonly structs/strings dont get serialized anymore
2. get only properties of structs/strings dont get serialized anymore
3. HasSet fix in fielddescriptor

Related Issue

https://discord.com/channels/500285081265635328/1156630820430303373
First part of fixing the serialization on get only members.
Fixing non backing fields is unaffected by this PR

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My change requires a change to the documentation.
• I have added tests to cover my changes.
• All new and existing tests passed. ( The Image tests and rasterized fonts test fail i was never able to get them working, but they fail without the changes too )

[Eideren]

Can I close this one @IXLLEGACYIXL ?

[IXLLEGACYIXL]

yes