Minimessage Web Editor Support

[Silthus]

okay, all of the API endpoints which accept json input can also take a placeholders field, like {"miniMessage": "...", "placeholders": {...} }
the actually contents of this placeholders object are three fields, stringPlaceholders, componentPlaceholders and miniMessagePlaceholders. All three are just key-value maps.
Note that componentPlaceholders are directly as a json object, while the others are strings ({"componentPlaceholders": {"text": "something", "color": "red"}, "miniMessagePlaceholders": "accepts tags!"})
miniMessagePlaceholders are parsed into components by the server