[Core] Implement Error Handler / Callback

[nicbarker]

This PR changes Clay_Initialize to require that the user provide a function that Clay can call when it encounters an error.
The current error types include:

typedef enum {
    CLAY_ERROR_TYPE_TEXT_MEASUREMENT_FUNCTION_NOT_PROVIDED,
    CLAY_ERROR_TYPE_ARENA_CAPACITY_EXCEEDED,
    CLAY_ERROR_TYPE_ELEMENTS_CAPACITY_EXCEEDED,
    CLAY_ERROR_TYPE_TEXT_MEASUREMENT_CAPACITY_EXCEEDED,
    CLAY_ERROR_TYPE_DUPLICATE_ID,
    CLAY_ERROR_TYPE_FLOATING_CONTAINER_PARENT_NOT_FOUND,
    CLAY_ERROR_TYPE_INTERNAL_ERROR,
} Clay_ErrorType;

This is part of work to allow the user to handle the case where clay runs out of space in its internal arena, and needs to reallocate with more memory.