LibWeb: Begin implementing the HTMLInputElement 'image' type state

This implements enough to represent <input type=image> with its loaded
source image (or fallback to its alt text, if applicable). This does not
implement acquring coordinates from user-activated click events on the
image.

Tests/LibWeb/Text/input/input-image.html

@@ -0,0 +1,31 @@
+<script src="./include.js"></script>
+<script type="text/javascript">
+    const SOURCES = ["../../Layout/input/120.png", "file:///i-do-no-exist-i-swear.png"];
+
+    const runTest = source => {
+        let input = document.createElement("input");
+        input.type = "image";
+        input.alt = "submit";
+
+        return new Promise((resolve, reject) => {
+            input.addEventListener("load", () => {
+                resolve(`${input.src} loaded`);
+            });
+            input.addEventListener("error", () => {
+                resolve(`${input.src} failed`);
+            });
+
+            input.setAttribute("src", source);
+        });
+    };
+
+    asyncTest(done => {
+        let promises = SOURCES.map(source => runTest(source));
+
+        Promise.allSettled(promises)
+            .then(results => results.map(result => result.value))
+            .then(results => results.sort())
+            .then(results => results.forEach(println))
+            .finally(done);
+    });
+</script>