add foundation for WebUSB-based install page

static/js/web-install.js

@@ -0,0 +1,32 @@
+// @license magnet:?xt=urn:btih:d3d9a9a6595521f9666a5e94cc830dab83b65699&dn=expat.txt MIT
+
+async function doConnect() {
+    const webusb = await Adb.open("WebUSB");
+
+    console.log("connected");
+
+    if (webusb.isAdb()) {
+        console.log("adb");
+        const adb = await webusb.connectAdb("host::");
+        await adb.reboot("bootloader");
+        return;
+    }
+
+    if (webusb.isFastboot()) {
+        console.log("fastboot");
+        const fastboot = await webusb.connectFastboot();
+        await fastboot.send("flashing unlock");
+        await fastboot.receive();
+    }
+}
+
+if ("usb" in navigator) {
+    console.log("WebUSB available");
+
+    const connect = document.getElementById("connect");
+    connect.onclick = doConnect;
+} else {
+    console.log("WebUSB unavailable");
+}
+
+// @license-end