bug#59745: 26.3; Rename `string-collate-lessp' to respect the convention

[Drew Adams]

(elisp) `Coding Conventions' tells us:

   If the purpose of a function is to tell you whether a certain
   condition is true or false, give the function a name that ends in
   'p' (which stands for "predicate").  If the name is one word, add
   just 'p'; if the name is multiple words, add '-p'.  Examples are
   'framep' and 'frame-live-p'.

`string-collate-lessp' should, according to convention, be renamed to
`string-collate-less-p', no?  Every such misnaming encourages others,
as people tend to base new names on names they see for existing things.

In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.19044
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''