Move internal packages to an internal folder

For reference: https://go.dev/doc/go1.4#internalpackages

internal/reader/sanitizer/truncate.go

@@ -0,0 +1,22 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package sanitizer
+
+import "strings"
+
+func TruncateHTML(input string, max int) string {
+	text := StripTags(input)
+	text = strings.ReplaceAll(text, "\n", " ")
+	text = strings.ReplaceAll(text, "\t", " ")
+	text = strings.ReplaceAll(text, "  ", " ")
+	text = strings.TrimSpace(text)
+
+	// Convert to runes to be safe with unicode
+	runes := []rune(text)
+	if len(runes) > max {
+		return strings.TrimSpace(string(runes[:max])) + "…"
+	}
+
+	return text
+}