[Koha-translate] Sinhala: need to know how plurals work and to define it in the translation platform config

[Victor Grousset/tuxayo]

Hi,

When releasing the latest Koha in the 21.05.x series, an automated check 
complains about nplurals not being defined.

in si-messages-js.po
"Plural-Forms: nplurals=INTEGER; plural=EXPRESSION;\n"
note: Plural-Forms seems to be only in this file and not in master, it's 
in 21.05.x branch.

@Sinhala speakers: how does plural work?
How many forms are there and what are the rules?
For example some languages have sigular and three plurals.
With the 1st plural form being used when there are two elements.
With the 2nd plural form being used when there are between 3 and 5 elements.
With the 3rd plural form being used when there are between 5 and more 
elements.

@Bernardo maybe there is somewhere already the info for .po file for 
Sinhala?
Or maybe we do like the other files and Koha branches where the 
Plural-Forms head isn't there? But how like plural translations work then?

Cheers,

-- 
Victor Grousset/tuxayo