Add hologram Converter API

[Matt-MX]

Adds the HologramConverter interface for creating multiple converter implementations.

e.g. a builtin DecentHologramsConverter that converts lines and optionally attempts to convert icons to item holograms that align with the text

Adds the FHConversionRegistry to store and procure all HologramConverter.
Adds the /fancyholograms convert <converterId> <target> [args...] command to invoke the converters.
Adds the ConverterTarget class which is essentially a regex to target holograms individually or as groups.

An example of API usage:

ConverterTarget target = ConverterTarget.all();

// The constructor can also take in a CommandSender sender  and String[] args for logging results/errors and processing additional arguments.
HologramConversionSession session = new HologramConversionSession(target);

FHConversionRegistry.getConverterById("DecentHolograms")
  .ifPresent((converter) -> {
    List<HologramData> data = converter.convert(session);

    // Handle the data yourself e.g. add it to manager and save.
  });

[Matt-MX]

This additionally allows players to control translation of holograms through the command /holo edit <hologram> translation <x> <y> <z>

[OliverSchlueter]

When I type "/fancyholograms convert DecentHolograms " and then press tab, it throws an exception "DecentHolograms holograms folder doesn't exist or is empty"
I probably wound't suggest DecentHolograms at all, when the folder is empty or doesn't exists.
There are a few TODO comments in the DecentHologramsConverter class, idk if you wanted to solve them.
LGTM otherwise

[Matt-MX]

I'll have a look at the exception and finish the TODOs, I think it's just for handling errors.

[Matt-MX]

@OliverSchlueter Should have fixed the thrown exception, also filters out any converters deemed not usable from tab completion.