[PLAT-5412] Add BugsnagStackframe.type

[nickdowell]

Goal

Adding type to BugsnagStackframe is a prerequisite for supporting mixed stack traces containing multiple languages.

Design

The type parameter is intended to be nil in most cases, and only set if the stack trace contains more than one language - e.g. a reactnativejs exception that contains some cocoa frames.

Because the existing BSGErrorType is an int-based enum and cannot naturally represent a nil value, a new Objective-C style string enum BugsnagStackframeType has been created specifically for this property.

In a future notifier update when we are making breaking API changes, we should consider switching all our enums that are serialized as strings to NS_TYPED_ENUMs.

Changeset

• Added type parameter
• Added BugsnagStackframe+Private.h to remove duplicate declarations
• Adopted NS_ASSUME_NONNULL for stack frame header

Testing

• Implemented unit tests to validate expected behaviour of type parameter
• Manually verified using a customised version of BugsnagReactNative

[kstenerud]

Would we also need enum values for react-native and other frameworks on top of ios?

[nickdowell]

No - this exists only so that BugsnagReactNative can set the type of native stack frames. The reactnativejs type is applied to the containing error, which specifies the type for all frames that don't explicitly state a type.