bug#72678: intraline whitespace not supported between backslash and newline in R6RS string literals

According to R6RS § 4.2.7 and R7RS § 6.7, intraline whitespace
(i.e. spaces and tabs) should be allowed between backslash and newline
in R6RS string literals.

Guile 3.0.10 does not support this.

How to reproduce:

$ tr -d '$' > foo.scm <<\EOF
(begin
  #!r6rs
  (display "Hello \$
           mama")
  (display "Hello \  $
           papa"))
EOF
$ guile foo.scm
=>
ice-9/read.scm:126:4: In procedure read-string:
/home/bruno/foo.scm:5:21: invalid character in escape sequence: #\space

$ guile --version
guile (GNU Guile) 3.0.10
...