[path/filepath] Change join() to take a []string instead of varargs

This makes passing an allocator easier, as you no longer have to resort to
named arguments:
Before:
  `join(a, b, c)` became `join(elems={a, b, c}, allocator=ally)`
After:
  `join({a, b, c})` becomes `join({a, b, c}, ally)`

core/path/filepath/path_windows.odin

@@ -88,7 +88,7 @@ abs :: proc(path: string, allocator := context.allocator) -> (string, bool) {
 }
 
 
-join :: proc(elems: ..string, allocator := context.allocator) -> string {
+join :: proc(elems: []string, allocator := context.allocator) -> string {
 	for e, i in elems {
 		if e != "" {
 			return join_non_empty(elems[i:], allocator)