Convert to TypeScript

This needs the usual "I know the `app` element exists" annotation, plus
dealing with a non-existing /etc/hostname (in which case the watch will
return `null`, and we shouldn't poke that into a `string` state).

tsconfig.json

@@ -0,0 +1,22 @@
+{
+  "compilerOptions": {
+    "allowJs": true,
+    "checkJs": true,
+    "exactOptionalPropertyTypes": true,
+    "jsx": "react",
+    "lib": [
+      "dom",
+      "es2020"
+    ],
+    "paths": {
+        "*": ["./pkg/lib/*"]
+    },
+    "moduleResolution": "bundler",
+    "noEmit": true,
+    "strict": true,
+    "target": "es2020"
+  },
+  "include": [
+    "src/**/*"
+  ]
+}