fix: Update Sinhala translations

[dinithija]

• README-si - Minor changes and improvements.
• https://github.com/iamkun/dayjs/blob/dev/src/locale/si.js
  • Fixed දිනය / දින (Date / Dates) -> දවස / දවස් (Day / Days)
  • We don't have short words for month names and I noticed that three letters are used to abbreviate month names in English locale/en-gb.js. So i decided to keep three letters. (otherwise දුරු = cummin, නව = new, ඇස = eye)

I noticed that the locale/en-gb.js code order is different. Should we follow it? If so, please amend accordingly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.73%. Comparing base (e363522) to head (0282479).
Report is 43 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #2766      +/-   ##
==========================================
- Coverage   99.77%   99.73%   -0.05%     
==========================================
  Files         183      184       +1     
  Lines        2262     2290      +28     
  Branches      642      646       +4     
==========================================
+ Hits         2257     2284      +27     
- Misses          5        6       +1     

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