Omit browserify-preprocessor from dependencies

[peterbee]

This reverts a previous change to add @cypress/browserify-preprocessor to this package's dependencies. It also clarifies usage in the README for Clojurescript-only and CLJS + JS environments.

⚠️ I had trouble testing this locally. I could not discern if my troubles were related to this change or just with npm linking.

[viesti]

Sorry, have been a bit lazy with this :/ Seems that since last summer when hacking on this, Cypress was on version 4.x, now it's on version 6.x (!). Also seems that version 5.0.0 switched to using Webpack preprocessor by default: https://docs.cypress.io/guides/references/changelog.html#5-0-0

Cypress no longer supports file paths with a question mark ? or exclamation mark ! in them. We now use the webpack preprocessor by default and it does not support files with question marks or exclamation marks. Addressed in #7982.

So it makes even more sense to remove the dependency to browserify preprocessor I guess :)

If I remember correctly, the problem was that if one does not delegate to the default preprocessor, the file is not preprocessed at all, which is a problem for even the sample code of a fresh cypress install, since the samples use code that requires preprocessing. So if a custom preprocessor doesn't handle a file, the fallback has to be explicitly written.

The makeBrowserifyPreprocessor() reference in the readme in this PR relies on makeBrowserifyPreprocessor being available. The original readme had a more full example, on installing the cypress/browserify-preprocessor dependency and making the helper available via

const makeBrowserifyPreprocessor = require('@cypress/browserify-preprocessor');

What is a bit funny that I now don't remember how one could fallback to use the default preprocessor bundled inside Cypress installation. Might be good to look if there is an instruction for that (maybe I missed) or if not, hop onto the Cypress 6.x time and even use the webpack preprocessor (https://github.com/cypress-io/cypress/tree/master/npm/webpack-preprocessor), since this is what new projects would probably use.

[viesti]

Hmm, haven't looked how other preprocessors work that much, but this made me wonder that is the conditional check even necessary, assuming that each preprocessor checks if a file should be processed: cypress-io/cypress#5832 (comment)