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/tsconnect.go

@@ -20,6 +20,7 @@ import (
 var (
 	addr            = flag.String("addr", ":9090", "address to listen on")
 	distDir         = flag.String("distdir", "./dist", "path of directory to place build output in")
+	pkgDir          = flag.String("pkgdir", "./pkg", "path of directory to place NPM package build output in")
 	yarnPath        = flag.String("yarnpath", "../../tool/yarn", "path yarn executable used to install JavaScript dependencies")
 	fastCompression = flag.Bool("fast-compression", false, "Use faster compression when building, to speed up build time. Meant to iterative/debugging use only.")
 	devControl      = flag.String("dev-control", "", "URL of a development control server to be used with dev. If provided without specifying dev, an error will be returned.")
@@ -37,6 +38,8 @@ func main() {
 		runDev()
 	case "build":
 		runBuild()
+	case "build-pkg":
+		runBuildPkg()
 	case "serve":
 		runServe()
 	default: