squash! Add so-long library

Revised log entry:

Add so-long library

* lisp/so-long.el: New library.
* doc/emacs/trouble.texi (Long Lines): New node covering so-long.el.
* doc/emacs/emacs.texi (Top): Add menu entry for the Long Lines node.
* etc/NEWS: Include under "New Modes and Packages in Emacs 27.1"

doc/emacs/emacs.texi

@@ -1176,6 +1176,7 @@ Dealing with Emacs Trouble
 * Crashing::            What Emacs does when it crashes.
 * After a Crash::       Recovering editing in an Emacs session that crashed.
 * Emergency Escape::    What to do if Emacs stops responding.
+* Long Lines::          Mitigating slowness due to extremely long lines.
 
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