Added Source converted to Package

[Selva1910]

Description

Converted the Unity Source into Package. So that, it can be added from git directly. And Added Overridable folder for builds. Right now for WebGL. Also, Refactored the Addressable uncomment approach with Symbols.

Can you check this @juicycleff

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore
  [new Feature] Load FlutterUnityIntegration package via git URL in package manager #328

[timbotimbo]

Hey, I think you've made some useful contributions here but it is really hard to view the changes.

Because you made your changes in a copy of the original files, everything in this PR just shows up as a new file.
But when I check the individual files, a lot seems to have actually changed inside them.

For example, I just pasted build.cs into an online diff tool and I see hundreds of changed lines.
From a quick look of build.cs:

• Added customized export paths + GUI
• Added addressables define logic
• Code formatting changes
• Removed il2cpp compiler setting for Unity 2022 but added it for 6000.
  It also ignores the debug/release option for 6000.
• Added spaces in css identifiers which likely breaks the actual css.
  #unity-container -> # unity-container

Your package and the Unity example now also contain different versions of the export scripts.

Could you somehow split these changes?

Maybe:

• Apply code changes in the Unity example and move that to unity_package.
• Or move the code from the Unity excample first and then apply your changes.

Afterwards you could probably import the package in the Unity example to avoid duplicating these scripts in the repo.

The unity 6000 branch also includes (minor) export changes to handle breaking changes in Unity 6.

It might be good if I ported those over to master before applying a major change like this.

Juicycleff isn't very active anymore, so this repo and the pub.dev plugin are nearly abandoned.
I try to do some minimal maintenance to keep it running, but I have limited permissions to the master branch.
So I can't quickly modify small issues possibly introduced by a PR.