Add partial reload support

[DerpyNewbie]

Expected to release after 0.6, but before 0.7.

• Gun provides some methods to handle reloading, but these are not tested since it always returns the same value.
• Add reloading system #185 is expected to release in 0.7.0, but it still requires lots of work, and may take more than a year for release.
• Adding simple guns-down reload system will open up many choices available to system users,
  and might be significant enough to change current VRChat airsoft scene.

Original idea came from talking to ルシファーさん talking about reloading system.

Specification

• Gun will have these properties
  • TotalBulletsRemaining
    • In reserve
  • CurrentBulletsRemaining
    • In magazine
  • InitialTotalBullets
  • ReloadTimeInSeconds
• ManagedGun's InitialTotalBullets can be configured when summoning
• TotalBulletsRemaining can be reset to InitialTotalBullets with some kind of methods

[Nuinomi]

リロードシステムの追加＃185　が追加されるまでの繋ぎの簡易的なリロードシステムという認識で問題ないですか？

[DerpyNewbie]

@Nuinomi その認識で問題ないです!