prebirth: Add `string-append' to libprebirth

* build-aux/bootstrap/libprebirth.js ($$string$_$append): Add function.

build-aux/bootstrap/libprebirth.js

@@ -89,6 +89,11 @@ const $$string$q$$7$    = ( x, y ) => x === y;
 const $$string$_$ref    = ( s, i ) => s[ i ]
                               || _error( `value out of range: ${i}`);
 
+function $$string$_$append()
+{
+    return argToArr( arguments ).join( "" );
+}
+
 // R7RS math
 function $$$p$()
 {