Make ?\LF generate 10, not -1 (bug#55738)

The old -1 value was an artefact of the reader implementation.

* src/lread.c (read_escape): Remove the `stringp` argument; assume
character literal syntax.  Never return -1.
(read_string_literal): Handle string-specific escape semantics here
and simplify.
* test/src/lread-tests.el (lread-escaped-lf): New test.
2 jobs for master in 34 minutes and 39 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build Images
failed #46680


skipped #46681
Name Stage Failure
build-image-inotify Build Images
executor failed running [/bin/sh -c make bootstrap]: exit code: 2
Running after_script
Running after script...
$ test -n "$(docker ps -aq -f name=${test_name})" && docker cp ${test_name}:checkout/test ${test_name}
$ test -n "$(docker ps -aq -f name=${test_name})" && docker rm ${test_name}
$ find ${test_name} ! \( -name "*.log" -o -name ${EMACS_TEST_JUNIT_REPORT} \) -type f -delete
find: build-image-inotify-52e527a0: No such file or directory
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1