LibWeb: Add TextEncoder encodeInto

Author: https://github.com/bplaat
Commit: 0104225d9b
Pull-request: https://github.com/SerenityOS/serenity/pull/21351

Tests/LibWeb/Text/input/TextDecoder/TextDecoder_decode.html

@@ -1,12 +0,0 @@
-<script src="../include.js"></script>
-<script>
-    test(() => {
-        try {
-            let decoder = new TextDecoder("utf-8");
-            println(`[${decoder.decode(new Uint8Array([0x41, 0x42, 0x43]))}]`); // "ABC"
-            println(`[${decoder.decode()}]`);
-        } catch(e) {
-            println("ERROR: " + e.name + ": " + e.message);
-        }
-    });
-</script>