[entity] Implement support for `target_smallprint`

[heysokam]

This will print a message on the center of the screen when triggered.
In Defrag, by default, only the activating client will see the message.

Keys

• message | Message | string | | Text string to print on screen.
• targetname | Target Name | targetname | | The activating trigger points to this.

Spawnflags

• BROADCAST | When set, the message will be broadcast to all connected players.