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/README.md

@@ -28,3 +28,13 @@ To serve them, run:
 ```
 
 By default the build output is placed in the `dist/` directory and embedded in the binary, but this can be controlled by the `-distdir` flag. The `-addr` flag controls the interface and port that the serve listens on.
+
+# Library / NPM Package
+
+The client is also available as an NPM package. To build it, run:
+
+```
+./tool/go run ./cmd/tsconnect build-pkg
+```
+
+That places the output in the `pkg/` directory, which may then be uploaded to a package registry (or installed from the file path directly).