Share the launcher rule of a TypeScript bundle, so builder and controller name it alike (R4e, the lead's request for dev-r4e-controller)

The rule mesh-build-node and build-agent each keep their own copy of: x.js is
started by x.serve.mjs beside it; other languages get none (ADR 0193).

go/buildstep/launchers.go

@@ -0,0 +1,24 @@
+package buildstep
+
+import "strings"
+
+// LauncherSuffix is what a TypeScript entrypoint's launcher is called beside it: index.js is started as
+// index.serve.mjs (novox/hq ADR 0193), an ES module by its own extension whatever the bundle's package.json says.
+const LauncherSuffix = ".serve.mjs"
+
+// Launchers are the launchers a bundle's build writes, by entrypoint, relative to the bundle's root: for TypeScript
+// every entrypoint `x.js` is started by `x.serve.mjs` beside it; an entrypoint that is not JavaScript and every other
+// language, whose build is already executable, get none. The one rule mesh-build-node, build-agent and the controller
+// share (R4e), so the name a build writes is the name a reader expects.
+func Launchers(language string, entrypoints []string) map[string]string {
+	out := map[string]string{}
+	if language != "typescript" {
+		return out
+	}
+	for _, entry := range entrypoints {
+		if strings.HasSuffix(entry, ".js") {
+			out[entry] = strings.TrimSuffix(entry, ".js") + LauncherSuffix
+		}
+	}
+	return out
+}

go/buildstep/launchers_test.go

@@ -0,0 +1,22 @@
+package buildstep
+
+import (
+	"reflect"
+	"testing"
+)
+
+// A TypeScript bundle's entrypoint x.js is started as x.serve.mjs beside it (novox/hq ADR 0193), the rule mesh-build-node
+// and build-agent both apply; an entrypoint that is not .js gets none, and another language gets none.
+func TestATypeScriptEntrypointIsStartedByTheLauncherBesideIt(t *testing.T) {
+	got := Launchers("typescript", []string{"index.js", "tools/x.js", "data.json"})
+	want := map[string]string{"index.js": "index.serve.mjs", "tools/x.js": "tools/x.serve.mjs"}
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("Launchers(typescript) = %v, want %v", got, want)
+	}
+	if got := Launchers("go", []string{"index.js"}); len(got) != 0 {
+		t.Fatalf("a Go bundle got launchers: %v", got)
+	}
+	if got := Launchers("typescript", nil); len(got) != 0 {
+		t.Fatalf("a bundle with no entrypoint got launchers: %v", got)
+	}
+}