os.UserConfigDir and os.UserCacheDir are missing from os package

[tchajed]

The tinygo standard library doesn't include os.UserConfigDir and [os.UserCacheDir](https://cs.opensource.google/go/go/+/refs/tags/go1.25.1:src/os/file.go;l=507}. This seems like a simple oversight given that os.UserHomeDir is implemented.

Minimal example:

package main

import "os"

func main() {
	_, _ = os.UserConfigDir()
}

$ tinygo build -o main main.go
# command-line-arguments
main.go:6:12: undefined: os.UserConfigDir