cmd/tsconnect: extract NPM package for reusing in other projects

`src/` is broken up into several subdirectories:
- `lib/` and `types`/ for shared code and type definitions (more code
  will be moved here)
- `app/` for the existing Preact-app
- `pkg/` for the new NPM package

A new `build-pkg` esbuild-based command is added to generate the files
for the NPM package. To generate type definitions (something that esbuild
does not do), we set up `dts-bundle-generator`.

Includes additional cleanups to the Wasm type definitions (we switch to
string literals for enums, since exported const enums are hard to use
via packages).

Also allows the control URL to be set a runtime (in addition to the
current build option), so that we don't have to rebuild the package
for dev vs. prod use.

Updates #5415

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

cmd/tsconnect/src/app/go-panic-display.tsx

@@ -0,0 +1,21 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+export function GoPanicDisplay({
+  error,
+  dismiss,
+}: {
+  error: string
+  dismiss: () => void
+}) {
+  return (
+    <div
+      class="rounded bg-red-500 p-2 absolute top-2 right-2 text-white font-bold text-right cursor-pointer"
+      onClick={dismiss}
+    >
+      Tailscale has encountered an error.
+      <div class="text-sm font-normal">Click to reload</div>
+    </div>
+  )
+}