Initial Implementation of Global for Prefix Benefits

[Solxanich]

What is the new feature?

#4184

Why should this be part of tModLoader?

To increase cross-mod compatibility and 'vanilla'-like behavior.

Are there alternative designs?

Could implement new GlobalPrefix instead of adding another hook to GlobalItem class

For one hook, I'm not sure it adds significant value to break out a net new global. There isn't exactly a 'Prefix' entity to apply this to in 'normal' fashion. And all the other 'Update Accessory' logic is already in GlobalItem.
On the flip side, it would be significantly more consistent with other hook usage & I assume more performant.

Sample usage for the new feature

              {
                  /* Prehardmode = 1
                   * Hardmode = 2
                   * Post-Moon Lord = 3
                   * Post-DoG = 4
                   */
  
                  if (DownedBossSystem.downedDoG)
                      self.statDefense += 3;
                  else if (NPC.downedMoonlord)
                      self.statDefense += 2;
                  else if (Main.hardMode)
                      self.statDefense += 1;
  
                  self.endurance += 0.0025f;
              }```

### ExampleMod updates