Add internal placeholders

[BlitzOffline]

Add internal placeholders that would work inside menus. One placeholder people seem to want a lot is a "slot" placeholder that would return the slot of the current item. Something to note is that not all placeholders would work everywhere. There would be menu placeholders and item placeholders.

[cj89898]

I was using $slot$ just FYI (randomly chosen format) when I was trying to develop the feature a while back.

[BlitzOffline]

Internal placeholder idea:
anvil_input - would allow menu designers use the text entered by players inside the anvil text box.

[BlitzOffline]

Internal placeholder idea:
has_placeholder_player - boolean (true/false) - would return true if the menu was open using /dm open <menu> [viewer] [-p:<target>] and a target is specified