Add CSharp BSGErrorType

[kstenerud]

Goal

Manually integrating bugsnag-cocoa into a Unity app requires a BSGErrorTypeCSharp enum.

Testing

Updated tests to check the csharp type.

[github-actions]

Bugsnag.framework binary size did not change - 707,312 bytes

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%     +92  +0.0%     +92    __TEXT,__text
  +0.2%     +32  +0.2%     +32    __DATA,__cfstring
  +0.0%      +7  +0.0%      +7    __TEXT,__cstring
  -0.4%     -32  -0.2%     -32    [__DATA]
  -0.4%     -99  -0.4%     -99    [__TEXT]
  [ = ]       0  [ = ]       0    TOTAL

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[tomlongridge]

LGTM - pending changelog