ci: add codecov config, publish on release

[jeroen-meijer]

• includes small readme and changelog revisions
• ci can now be manually triggered

[codecov]

Codecov Report

Merging #5 into master will increase coverage by 1.585%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##            master        #5       +/-   ##
=============================================
+ Coverage   98.000%   99.585%   +1.585%     
=============================================
  Files            7         6        -1     
  Lines          250       241        -9     
=============================================
- Hits           245       240        -5     
+ Misses           5         1        -4     

Impacted Files	Coverage Δ
lib/src/models/romanization/romanization.dart

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