[PLAT-8318] Fix inaccurate isPC & isLR values

[nickdowell]

Goal

Improve the accuracy of isPC and isLR stackframe values which are used by the back-end to determine how to symbolicate frameAddress.

Changeset

Examines the register values captured by KSCrash to compute isPC and isLR values in +[BugsnagThread enhanceThreadInfo:].

Switches to using the BSG_KSCrashField_* keys in BugsnagStackframe.m.

Improves header documentation in BugsnagStackframe.h.

Testing

Amends existing unit tests to verify isPC and isLR are set when expected.

Amends existing E2E scenarios to verify isPC and isLR.

[github-actions]

Bugsnag.framework binary size decreased by 72 bytes from 758,152 to 758,080 🎉

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +1.1%    +292  +1.1%    +292    [__TEXT]
  +0.5%     +96  +0.5%     +96    __DATA,__cfstring
  [ = ]       0  +0.6%     +72    [__LINKEDIT]
  -1.2%     -31  -1.2%     -31    __TEXT,__objc_methtype
  -0.1%     -37  -0.1%     -37    __TEXT,__objc_methname
  -0.0%     -72  -0.0%     -72    String Table
  -0.7%     -96  -2.1%     -96    [__DATA]
  -0.1%    -224  -0.1%    -224    __TEXT,__text
  -0.0%     -72  [ = ]       0    TOTAL

Generated by 🚫 Danger