Accept /[\-]/u as a valid regular expression (#288)

The non-Unicode version of the pattern was already accepted.

test262 tests it in an inverted sense in
test/built-ins/RegExp/unicode_restricted_identity_escape.js but
it appears to be per spec and both V8 and Spidermonkey accept it.

Fixes: https://github.com/quickjs-ng/quickjs/issues/286

libregexp.c

@@ -783,6 +783,9 @@ static int get_class_atom(REParseState *s, CharRange *cr,
                     /* always valid to escape these characters */
                     goto normal_char;
                 } else if (s->is_unicode) {
+                    // special case: allowed inside [] but not outside
+                    if (ret == -2 && *p == '-' && inclass)
+                        goto normal_char;
                 invalid_escape:
                     return re_parse_error(s, "invalid escape sequence in regular expression");
                 } else {

tests/test_builtin.js

@@ -699,6 +699,9 @@ function test_regexp()
         ex = _ex;
     }
     assert(ex?.message, "invalid class range");
+
+    eval("/[\\-]/");
+    eval("/[\\-]/u");
 }
 
 function test_symbol()