feat: summarize diffs that only change line endings

[dsherret]

With --json (used when annotations are enabled), dprint defaults to the unified diff format, which doesn't have the "Text differed by line endings." shortcut that the pretty format has. So a CRLF checkout on Windows produced a full diff of every line in every file, each with a \r marker, plus an annotation spanning the whole file.

This makes the annotate script detect when a diff only changes line endings and:

• print "Text differed by line endings." instead of the diff, matching dprint's plain output
• point the annotation at line 1 instead of highlighting the whole file
• when running on a Windows runner and the checked out file had CRLF line endings, add a hint to the annotation recommending only running the action on Linux, with a link to the README's Windows line endings section

A file that only gains a final newline is still shown as a full diff, since that's a real change rather than a line ending one.