feat: support tilde in url

From Roberta Calegari <roberta.calegari@unibo.it>

May I kindly ask you also if it is possible to insert the  ~ in the url of authors command?
We have a problem with the authors urls in the new format: the tilde ~ symbol that is interpreted in a very strange way, generating a bad url, and it's not clear to me how to solve the issue.

\author[4]{Amal {El Fallah-Seghrouchni}}[%
orcid=0000-0002-8390-8780,
email=amal.elfallah@lip6.fr,
url={https://webia.lip6.fr/~elfallah/},
]

Produces the url https://webia.lip6.fr/\protect\unhbox\voidb@x\protect\penalty\@M\{}elfallah/

I tried with url=https://webia.lip6.fr/\%7Eelfallah/, but it prints the %7E symbols, I tried with \texttildelow,  \~, ~{}, and many other options, but some problems are still there.

tex/latex/ceurart/ceurart.cls

@@ -1020,7 +1020,8 @@
   plus      .tl_set:N   = \l_ceur_au_gplus_tl      ,
   gplus     .tl_set:N   = \l_ceur_au_gplus_tl      ,
   email     .tl_set:N   = \l_ceur_au_email_tl      ,
-  url       .tl_set:N   = \l_ceur_au_url_tl        ,
+  %% prevent URL expansion (due ~ sign)
+  url       .code:n   = { \tl_set:Nn \l_ceur_au_url_tl { \exp_not:n { #1 } } },
 }
 
 \keys_set:nn { ceur / author } {
@@ -2863,3 +2864,9 @@
 %
 % End of class 'ceurart'
 %
+
+%%% Local Variables:
+%%% mode: latex
+%%% TeX-master: t
+%%% coding: utf-8-unix
+%%% End: