fix: restore boot by updating Babel config and graphql-yoga v5 API

[mstuart]

npm start has been broken. Two separate bugs, both from dependency bumps that landed without corresponding code changes.

1. Babel config

.babelrc set useBuiltIns: 'usage' and debug, both removed in @babel/preset-env 8. require('@babel/register') threw before any app code ran:

[BABEL] The 'useBuiltIns' option has been removed. Please use babel-plugin-polyfill-corejs3 instead.

Dropped both options. targets: { node: 'current' } already means no polyfilling is needed here.

2. graphql-yoga v5 API

Fixing Babel exposed the real bug: graphql-yoga was bumped to ^5 in #35, which removed the v1 GraphQLServer constructor, so the app died with GraphQLServer is not a constructor. Ported server.js to the v5 createYoga + createSchema API served by node:http, keeping the workshop's teaching comments intact.

Verification

$ npm start
Server is running on http://localhost:4000/graphql

{hello}                 -> {"data":{"hello":"Hello World"}}
{hello(name:"Mark")}    -> {"data":{"hello":"Hello Mark"}}
GET /graphql (html)     -> 200 (GraphiQL loads)

README updated for the /graphql path and the Playground -> GraphiQL rename. Prettier clean.