Proposal: rename pluginsFile to backgroundFile

[chrisbreiding]

The term pluginsFile is somewhat of a misnomer, since it is exclusive to the background node process, but a plugin could be created that solely affects the driver and events in the browser.

We should rename the pluginsFile to the backgroundFile to make it clear it runs in the background process. We should also update the documentation to make it clear plugins can be implemented for the browser in the supportFile, for the background in the backgroundFile, or both.

Code-wise, we should deprecate the pluginFile config option and log a warning that the user should rename it to backgroundFile. Then in a future version, remove pluginsFile entirely.

[chrisbreiding]

Need to update references to and use of plugins in the following repos:

• https://github.com/cypress-io/cypress-webpack-preprocessor
• https://github.com/cypress-io/cypress-watch-preprocessor
• https://github.com/cypress-io/cypress-browserify-preprocessor
• All cypress-example-* repos: https://github.com/cypress-io?utf8=%E2%9C%93&q=example&type=&language=

[chrisbreiding]

The code for this is complete, but not yet released. It will be released in 4.0.

[jennifer-shehane]

The work for this was never released.

[jennifer-shehane]

Closing due to recent changes we've decided to make for the configuration file.