Add rollup-plugin-istanbul to the README

[elchininet]

I saw a mention to this package in this Playwright issue but as I saw in the README that it was intended to be used with babel-plugin-istanbul and I was not using it, I skipped it and used a copy of the baseFixtures.ts file directly. Later I noticed that this package is indeed the same code contained in that file with some extra features for env variables, so I replaced the baseFixtures.ts file with it.

This pull request adds information to the README of the repo that the code should be instrumented with istanbul-lib-instrument but to do that one could use plugins that use this library to perform such a task, like babel-plugin-istanbul or rollup-plugin-istanbul, so other users that are using the later to build their bundles could know that this package could be used to generate the coverage of the code with Playwright.

In one of the packages that I maintain, I use rollup to build the final bundle, and to instrument the code I use rollup-plugin-istanbul. As I am using Playwright with Docker, I cannot use nyc to start the server, or what nyc will capture will be the usage of the playwright.config.ts file. I needed to use the report option of nyc once the coverage files were generated in the .nyc_output folder. I also I addded a mention to this in the README to avoid giving the idea that running a server with nyc is the only option to generate the final coverage.