fix: Correct `dayjs.extend`'s return type

[melusc]

dayjs.extend returns dayjs (lowercase d), not Dayjs.

This means TypeScript allows chaining extend calls like dayjs.extend(...).extend(...) instead of having to do dayjs.extend(...); dayjs.extend(...).

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (f0c9a41) 99.77% compared to head (0690a4d) 100.00%.

:exclamation: Current head 0690a4d differs from pull request most recent head c9ab4f3. Consider uploading reports for the commit c9ab4f3 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##              dev     #1666      +/-   ##
===========================================
+ Coverage   99.77%   100.00%   +0.22%     
===========================================
  Files         183       179       -4     
  Lines        2262      1996     -266     
  Branches      641       507     -134     
===========================================
- Hits         2257      1996     -261     
+ Misses          5         0       -5     

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[melusc]

@iamkun

Not to bother you, but is there anything keeping this simple change from being merged? Just cause it's been open for two years now.