melt_quanteda_dict: Convert a quanteda dictionary to a long data.table format
Description
This is used internally in the tCorpus dictionary search functions, but can be used manually
for more control. For example, adding numeric scores for sentiment dictionaries, and specifying which label/code to use in search_dictionary().
The name of the column with the label/code. If dictionary contains multiple levels,
additional columns are added with the suffix _l[i], where [i] is the level.