[WIP] Fix clientSideOnly handling in Forge mod's toml

[Copilot]

• Explore repository structure and understand the code
• Understand the issue: clientSideOnly field needs to be added to ForgeModTOMLData interface
• Add clientSideOnly field to ForgeModTOMLData interface
• Parse clientSideOnly from root of TOML in readForgeModToml function
• Add serverSideOnly field for consistency (optional root-level field)
• Create test case to verify the changes
• Test the implementation

Original prompt

---

This section details on the original issue you should resolve

<issue_title>clientSideOnly ignored when root in Forge mod's toml</issue_title>
<issue_description>Some mods (Entityculling for example) set clientSideOnly=true as a root field on the mods.toml file. This can be attached to the ForgeModTOMLData and can be used to detect some client only mods.

EntityCulling's mods.toml:

modLoader="javafml"
loaderVersion="[28,)"
issueTrackerURL="https://github.com/tr7zw/EntityCulling/issues"
clientSideOnly=true

license="tr7zw Protective License"
[[mods]]
modId="entityculling"
version="1.8.2"
displayName="EntityCulling"
displayURL="https://modrinth.com/mod/entityculling"
logoFile="icon.png"
authors="tr7zw"
description='''
This mod uses async path-tracing to hide Tiles/Entities that are not visible.
'''
[[dependencies.entityculling]]
    modId="minecraft"
    mandatory=true
    versionRange="[1.21.6,)"
    ordering="NONE"
    side="BOTH"
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

Fixes #328

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.