builtin/credential-store: move is_rfc3986_unreserved to url.[ch]

builtin/credential-store: move is_rfc3986_unreserved to url.[ch]

is_rfc3986_unreserved() was moved to credential-store.c and was made
static by f89854362c (credential-store: move related functions to
credential-store file, 2023-06-06) under a correct assumption, at the
time, that it was the only place using it.

However now we need it to apply URL-encoding to submodule names when
constructing gitdir paths, to avoid conflicts, so bring it back as a
public function exposed via url.h, instead of the old helper path
(strbuf), which has nothing to do with 3986 encoding/decoding anymore.

This function will be used in subsequent commits which do the encoding.

Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>