feat: PlaceholderAPI expansion for streamer mode

[iLynxcat]

• Adds a PlaceholderAPI integration for registering placeholders.
• Adds a streamer mode placeholder, relational so it's only visible to players with permission to view it.
  • Use in placeholder-supported contexts: %rel_streamermode_tag%, %rel_streamermode_prefix%, %rel_streamermode_suffix%
• Adds PlaceholderAPI and TAB to development test server with default config including placeholder
• Adds LuckPerms' Folia build to development test server