rewrite html generator with html/template

internal/disk_usage/disk_usage_other.go

@@ -0,0 +1,9 @@
+//go:build !(linux || darwin || freebsd || windows)
+
+// Fallback, so the program still builds on other OSes.
+
+package disk_usage
+
+func diskUsage(path string) (Usage, error) {
+	return Usage{}, ErrNotSupported
+}