docs: remove path(/dev.html) from local development GraphiQL URL

[hxrxchang]

When I first tried to set up the development environment for GraphiQL, I noticed that the documentation in DEVELOPMENT.md mentioned that GraphiQL is hosted at http://localhost:8080/dev.html
However, it was actually hosted at http://localhost:8080, so I updated the documentation to reflect this.

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[acao]

@hxrxchang it looks like a dev tooling regression, as the /dev path is meant to resolve the unminified bundle for debugging

[hxrxchang]

@acao
Thanks for your review.
If you want to use dev.html as the entry point when in the development environment (i.e., when the server is started with webpack-dev-server), then it seems that the part at https://github.com/hxrxchang/graphiql/blob/fix-local-development-url/packages/graphiql/resources/webpack.config.js#L85 should be isHMR instead of !isHMR.
isHMR refers to process.env.WEBPACK_SERVE, which is an environment variable that webpack-dev-server sets automatically. Refer to https://github.com/webpack/webpack-dev-server/blob/a11d81f063585a80a427d2d9dfc79f58799278cc/lib/Server.js#L241-L243.

[acao]

@hxrxchang yes, it seems the local development regression was introduced the last time I upgraded webpack

it is working on preview release though:

https://graphiql-test.netlify.app/dev

as you can see, the code is bundled but not minified, as designed, and the react development versions are used for debugging, however, it seems the js and css source maps are not deploying to netlify