Error if reserved message keys are mixed with unreserved keys

[nicksnyder]

Makes it an error to provide data that mixes reserved message keys with unreserved ones. Previously the unreserved keys would be ignored, but this caused surprising behavior where if someone added a message that had a key with a reserved word, it would silently break the whole translation file. This change turns that situation into an error.

Resolves #209

[codecov]

Codecov Report

Attention: Patch coverage is 92.18750% with 5 lines in your changes missing coverage. Please review.

Project coverage is 75.73%. Comparing base (0a3a706) to head (09888bc).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
i18n/message.go	94.33%	2 Missing and 1 partial ⚠️
i18n/parse.go	81.81%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #320      +/-   ##
==========================================
+ Coverage   74.90%   75.73%   +0.82%     
==========================================
  Files          20       20              
  Lines        1634     1677      +43     
==========================================
+ Hits         1224     1270      +46     
+ Misses        347      344       -3     
  Partials       63       63              

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