Add relative locales to czech language

[NikolaRusakov]

Add czech language locale fuctionality

[codecov-io]

Codecov Report

Merging #312 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #312   +/-   ##
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  Coverage     100%   100%           
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  Files          32     33    +1     
  Lines         386    389    +3     
  Branches       53     53           
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+ Hits          386    389    +3

Impacted Files	Coverage Δ
src/locale/cs.js	100% <100%> (ø)

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[prantlf]

Coincidentally, I added the Czech locale too :-) As a part of PR #305 with the plugin called LocalisableFormat.

Differences to this PR:

• The key "s" translated as "several seconds" and not "second".
• Keys for the single value (s, m, h, d, M, Y) translated with the correct declension:
  • if used in the expression for the future.
  • if used in the expression with duration; only if the duration is used in accusative - in the expression "takes ..."., for example

In my case, expressions for the past cannot be translated correctly,because the preposition needs a different declension. In your case, you decided to translate correctly expressions of the past (although moving the preposition "before" after the value feels weird) and durations in nominative.

I opened issue #302 to explain and discuss the problem of the correct localisation. I sent PR #304, which localises the three scenarios (duration, future and past) with three different texts, optionally, if needed in a particular locale. I updated the Czech locale in that PR accordingly.

[greegus]

@NikolaRusakov beware that the weekdays list should be indexed from Sunday, not Monday.
https://github.com/iamkun/dayjs/blob/master/src/locale/de.js

[stepanh]

Czech language requires more complex rules as highlighted by @prantlf above

[iamkun]

I'll merge czech language locale #461, cause weekdays list should be indexed from Sunday.

BTW, @stepanh I'll do it later to support more complex rules like prantlf mentioned above.

[stepanh]

cc @prantlf