cmd/tsconnect: add Tailwind CSS support and switch to using it

Integrates Tailwind CSS as an esbuild plugin that invokes the CLI
to process the input. It takes ~400ms, so it seems like the easiest
option (vs running a separate process for dev mode).

Existing minimal look and feel is replicated with Tailwind classes,
mostly to prove that the entire system works, including unused
class removal.

Also fixes yarn warnings about package.json not having a license
(which were showing up when invoking any scripts).

Fixes #5136
Fixes #5129

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

cmd/tsconnect/common.go

@@ -13,6 +13,7 @@ import (
 	"path/filepath"
 	"runtime"
 	"strconv"
+	"time"
 
 	esbuild "github.com/evanw/esbuild/pkg/api"
 )
@@ -44,6 +45,12 @@ func commonSetup(dev bool) (*esbuild.BuildOptions, error) {
 		LogLevel:    esbuild.LogLevelInfo,
 		Define:      map[string]string{"DEBUG": strconv.FormatBool(dev)},
 		Target:      esbuild.ES2017,
+		Plugins: []esbuild.Plugin{{
+			Name: "tailscale-tailwind",
+			Setup: func(build esbuild.PluginBuild) {
+				setupEsbuildTailwind(build, dev)
+			},
+		}},
 	}, nil
 }
 
@@ -111,3 +118,30 @@ type EsbuildMetadata struct {
 		EntryPoint string `json:"entryPoint,omitempty"`
 	} `json:"outputs,omitempty"`
 }
+
+func setupEsbuildTailwind(build esbuild.PluginBuild, dev bool) {
+	build.OnLoad(esbuild.OnLoadOptions{
+		Filter: "./src/index.css$",
+	}, func(args esbuild.OnLoadArgs) (esbuild.OnLoadResult, error) {
+		start := time.Now()
+		yarnArgs := []string{"--silent", "tailwind", "-i", args.Path}
+		if !dev {
+			yarnArgs = append(yarnArgs, "--minify")
+		}
+		cmd := exec.Command(*yarnPath, yarnArgs...)
+		tailwindOutput, err := cmd.Output()
+		log.Printf("Ran tailwind in %v\n", time.Since(start))
+		if err != nil {
+			if exitErr, ok := err.(*exec.ExitError); ok {
+				log.Printf("Tailwind stderr: %s", exitErr.Stderr)
+			}
+			return esbuild.OnLoadResult{}, fmt.Errorf("Cannot run tailwind: %w", err)
+		}
+		tailwindOutputStr := string(tailwindOutput)
+		return esbuild.OnLoadResult{
+			Contents: &tailwindOutputStr,
+			Loader:   esbuild.LoaderCSS,
+		}, nil
+
+	})
+}