Fix a bug about nexus.input.isKeyRepeat

[FIX] Fixed nexus.input.isKeyRepeat when re-pressing after releasing.
Refactor the entry script.

[MOD] Move some functions from main.lua to src/game.lua file.
Refactor the code.

[MOD] Change nexus.screen to nexus.scene.
[MOD] Rename draw to render for most objects.
[MOD] Move nexus.manager.resource to nexus.resource.
[MOD] Now the scenes are managerd by nexus.game directly.
[MOD] Now the update and render are managed by scene itself, not
globally.
[DEL] Remove nexes.manage.object.
[DEL] Remove nexus.manage.screen.
[DEL] Remove nexus.manage.window.