added first third of the night to SunnahTimes

[magondev]

I added the first third of the night as a field in SunnahTimes, since there are many reports that its a sunnah to delay his ishaa-prayer to the first third of the night, see this hadith for example.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.55%. Comparing base (d4e4f04) to head (e5ce8c2).

Files	Patch %	Lines
lib/src/sunnah_times.dart	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
- Coverage   99.66%   99.55%   -0.12%     
==========================================
  Files          31       31              
  Lines         890      893       +3     
==========================================
+ Hits          887      889       +2     
- Misses          3        4       +1     

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[iamriajul]

@magondev Can you raise an issue here so we can discuss this in more depth?: https://github.com/batoulapps/Adhan/issues