Figure out the meaning of the item note byte in fumen files

[vivaria]

In KatieFrogs' fumen parser, she parsed a specific note byte as item:

tja2fumen/src/tja2fumen/parsers.py

Line 412 in 417bfe5

# - 'i': item

This byte has no parallel in TJA files, so I'm not sure how to set this byte when writing converted fumen data.

[vivaria]

When inspecting a random fumen file (Haikei Doppelganger Oni), I notice that the item byte follows a pattern.

• The first note of specific measures (11, 30, 49, 68) is assigned item: 1
• These measures start at 11 and increment 19 each time

The item value seems to be independent of the actual note value... so if there's a consistent pattern here, this should be easy to implement. I just need to check other fumens.