[TypeScript] Allow port to be a string (#72)

Fixing a regression spotted in pnpm: pnpm/pnpm#1905

The port in the Node.js URL object is a string, so it is more convenient to
use string for the port.

https://nodejs.org/api/url.html#url_url_port

index.d.ts

@@ -4,7 +4,7 @@ declare module 'https-proxy-agent' {
     namespace HttpsProxyAgent {
         interface HttpsProxyAgentOptions {
             host: string
-            port: number
+            port: number | string
             secureProxy?: boolean
             headers?: {
                 [key: string]: string