Add PCRE2_ALT_BSUX to pcre pattern compilation

This is the pcre2 equivalent of the PCRE_JAVASCRIPT_COMPAT flag we used
previously. Compare bullet point 4 of the pcre2 compatibility docs:
https://www.pcre.org/current/doc/html/pcre2compat.html
to bullet point 5 of the pcre1 compatibility docs:
https://www.pcre.org/original/doc/html/pcrecompat.html

src/std/regexp.c

@@ -49,7 +49,7 @@ HL_PRIM ereg *hl_regexp_new_options( vbyte *str, vbyte *opts ) {
 	size_t error_offset;
 	pcre2_code *p;
 	uchar *o = (uchar*)opts;
-	int options = PCRE2_UCP | PCRE2_UTF;
+	int options = PCRE2_ALT_BSUX | PCRE2_UCP | PCRE2_UTF;
 	while( *o ) {
 		switch( *o++ ) {
 		case 'i':