Add mailbox icon (#1000)

Update icons/mailbox.svg

Co-authored-by: Karsa <contact@karsa.org>

icons/mailbox.json

@@ -0,0 +1,13 @@
+{
+  "$schema": "../icon.schema.json",
+  "tags": [
+    "emails",
+    "messages",
+    "letters",
+    "mailing list",
+    "newsletter"
+  ],
+  "categories": [
+    "mail"
+  ]
+}

icons/mailbox.svg

@@ -0,0 +1,16 @@
+<svg
+  xmlns="http://www.w3.org/2000/svg"
+  width="24"
+  height="24"
+  viewBox="0 0 24 24"
+  fill="none"
+  stroke="currentColor"
+  stroke-width="2"
+  stroke-linecap="round"
+  stroke-linejoin="round"
+>
+  <path d="M22 17a2 2 0 0 1-2 2H4a2 2 0 0 1-2-2V9.5C2 7 4 5 6.5 5H18c2.2 0 4 1.8 4 4v8Z" />
+  <polyline points="15,9 18,9 18,11" />
+  <path d="M6.5 5C9 5 11 7 11 9.5V17a2 2 0 0 1-2 2v0" />
+  <line x1="6" y1="10" x2="7" y2="10" />
+</svg>