This generally seems like a better name, especially if we somehow also
need a better name for "read the entire buffer, but not the entire file"
somewhere down the line.
We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
This prevents us from needing a sv suffix, and potentially reduces the
need to run generic code for a single character (as contains,
starts_with, ends_with etc. for a char will be just a length and
equality check).
No functional changes.
This behaves very much like the regular diff command, showing the
start lines and ranges of additions/changes/deletions in both the
source and target files.
