Rename $count to $chunkSize in each()/eachById() for clarity on chunking behavior

[Jehong-Ahn]

The each() internally uses the chunk(). However, the variable name $count used as an argument in each() can be misleading to users unfamiliar with the internals of Eloquent. Many may incorrectly assume that $count refers to the iteration count, when in fact it controls the chunk size.

To improve clarity, I propose renaming the $count variable to $chunkSize. This change would make it clear that the argument defines the size of the chunk used internally by chunk(), reducing potential confusion.

I am attaching a tweet from a user who has also misunderstood the $count parameter in the each() method.