imporove release workflow ( installing plugins is now easier than before)

[KirisameMarisa]

• Unity
  • Created .unitypackage
  • Added unity project for the above
  • Included flatbuffers.dll
• Godot
  • Multi-platform support
• UnrealEngine
  • Included flatbuffer .h/.lib in UnrealEngine

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (8cac813) 30.29% compared to head (7c2cd12) 30.29%. Report is 1 commits behind head on master.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #89 +/- ## ======================================= Coverage 30.29% 30.29% ======================================= Files 99 99 Lines 2862 2862 Branches 388 388 ======================================= Hits 867 867 Misses 1940 1940 Partials 55 55 ```

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