Per-NPC Visibility Distance

[Grabsky]

This PR adds new command: /npc visibility_distance (npc) (value)

The last command argument can accept following values:

• Positive, whole number of blocks NPC will be visible from.
• default: Value specified in config,yml will be used.
• not_visible: NPC will be no longer visible.
• always_visible: NPC will be visible from all distances.

Changes to default translations file:

• Update output of /npc info command to include information about current visibility distance.
• Added messages.always_visible which is displayed in /npc info sub-command when NPC's visibility distance is set to always_visible.
• Added messages.not_visible which is displayed in /npc info sub-command when NPC's visibility distance is set to not_visible.
• Added messages.default which is currently only displayed in /npc info sub-command when NPC's visibility distance is set to default. This can be potentially used in various other places in the future. Placeholder {value} would return a context-aware default value.
• Added syntax and help entry for the new sub-command.
• Added output messages for the new sub-command.

Technical

Internally, special literals are mapped to following values:

• default is mapped to -1
• not_visible is mapped to 0
• always_visible is mapped to the output of Integer.MAX_VALUE (2147483647)