webpack-preprocessor: Add webpack 5 as peerDependency

[WIStudent]

Experimental support for webpack 5 was added in #16493, but the peerDependency in package.json currently only allows weback 4, resulting in the following error when trying to install webpack-preprocessor together with webpack 5 using npm@7:

$ npm install --save-dev @cypress/webpack-preprocessor
npm ERR! code ERESOLVE
npm ERR! ERESOLVE unable to resolve dependency tree
npm ERR!
npm ERR! While resolving: my-project@1.0.0
npm ERR! Found: webpack@5.39.0
npm ERR! node_modules/webpack
npm ERR!   dev webpack@"^5.39.0" from the root project
npm ERR!   peer webpack@">=2" from babel-loader@8.2.2
npm ERR!   node_modules/babel-loader
npm ERR!     dev babel-loader@"^8.2.2" from the root project
npm ERR!     peer babel-loader@"^8.0.2" from @cypress/webpack-preprocessor@5.9.0
npm ERR!     node_modules/@cypress/webpack-preprocessor
npm ERR!       dev @cypress/webpack-preprocessor@"*" from the root project
npm ERR!
npm ERR! Could not resolve dependency:
npm ERR! peer webpack@"^4.18.1" from @cypress/webpack-preprocessor@5.9.0
npm ERR! node_modules/@cypress/webpack-preprocessor
npm ERR!   dev @cypress/webpack-preprocessor@"*" from the root project

[cypress-bot]

The code for this is done in cypress-io/cypress#15611, but has yet to be released.
We'll update this issue and reference the changelog when it's released.

[chrisbreiding]

Released in 7.6.0.

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Cypress v7.6.0, please open a new issue.