Between Utils.prepend_line_number() and its only caller, there is a lot
of conversions from list to string to list and back to string. Simplify
all of this by making it accept and return a list of strings, which
avoids unnecessary splitting and joining.
The implementation was updated to use vim.iter():map() and
string.format() for better performance and to align with the idiomatic
functional style used elsewhere in the project.
The name of the function has also been tweaked and is now
"prepend_line_numbers()" to better reflect that it operates on a list of
strings.
The caller has been updated to match the new function signature.
638d237b75