cmd/tsconnect: switch to TypeScript

Continues to use esbuild for development mode and building. Also
includes a `yarn lint` script that uses tsc to do full type checking.

Fixes #5138

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

cmd/tsconnect/tsconfig.json

@@ -0,0 +1,13 @@
+{
+  "compilerOptions": {
+    "target": "ES2017",
+    "module": "ES2020",
+    "moduleResolution": "node",
+    "isolatedModules": true,
+    "strict": true,
+    "forceConsistentCasingInFileNames": true,
+    "sourceMap": true
+  },
+  "include": ["src/**/*"],
+  "exclude": ["node_modules"]
+}