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/common.go

@@ -17,6 +17,7 @@ import (
 	"time"
 
 	esbuild "github.com/evanw/esbuild/pkg/api"
+	"golang.org/x/exp/slices"
 )
 
 const (
@@ -38,7 +39,7 @@ func commonSetup(dev bool) (*esbuild.BuildOptions, error) {
 	}
 
 	return &esbuild.BuildOptions{
-		EntryPoints: []string{"src/index.ts", "src/index.css"},
+		EntryPoints: []string{"src/app/index.ts", "src/app/index.css"},
 		Outdir:      *distDir,
 		Bundle:      true,
 		Sourcemap:   esbuild.SourceMapLinked,
@@ -67,6 +68,47 @@ func commonSetup(dev bool) (*esbuild.BuildOptions, error) {
 	}, nil
 }
 
+// cleanDir removes files from dirPath, except the ones specified by
+// preserveFiles.
+func cleanDir(dirPath string, preserveFiles ...string) error {
+	log.Printf("Cleaning %s...\n", dirPath)
+	files, err := os.ReadDir(dirPath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return os.MkdirAll(dirPath, 0755)
+		}
+		return err
+	}
+
+	for _, file := range files {
+		if !slices.Contains(preserveFiles, file.Name()) {
+			if err := os.Remove(filepath.Join(dirPath, file.Name())); err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+func runEsbuild(buildOptions esbuild.BuildOptions) esbuild.BuildResult {
+	log.Printf("Running esbuild...\n")
+	result := esbuild.Build(buildOptions)
+	if len(result.Errors) > 0 {
+		log.Printf("ESBuild Error:\n")
+		for _, e := range result.Errors {
+			log.Printf("%v", e)
+		}
+		log.Fatal("Build failed")
+	}
+	if len(result.Warnings) > 0 {
+		log.Printf("ESBuild Warnings:\n")
+		for _, w := range result.Warnings {
+			log.Printf("%v", w)
+		}
+	}
+	return result
+}
+
 // setupEsbuildWasmExecJS generates an esbuild plugin that serves the current
 // wasm_exec.js runtime helper library from the Go toolchain.
 func setupEsbuildWasmExecJS(build esbuild.PluginBuild) {
@@ -167,7 +209,7 @@ type EsbuildMetadata struct {
 
 func setupEsbuildTailwind(build esbuild.PluginBuild, dev bool) {
 	build.OnLoad(esbuild.OnLoadOptions{
-		Filter: "./src/index.css$",
+		Filter: "./src/.*\\.css$",
 	}, func(args esbuild.OnLoadArgs) (esbuild.OnLoadResult, error) {
 		start := time.Now()
 		yarnArgs := []string{"--silent", "tailwind", "-i", args.Path}