Append extra text in documents as titles in the `Collection` loader.

[codetalker7]

Similar to how Python's ColBERT loads collections, a functionality should be added in the loader for Collection wherein extra text in documents is appended in front of the document as the document title. Currenly, the Collection loader only splits a line of the .tsv file once with the \t delimiter.

[codetalker7]

Update: we don't use Collections anymore (just a simple Vector{String}). However, the issue still remains.