fix(postcss-minify-selectors): quotes are removed even when attribute selector escaping was required

[ota-meshi]

Fix #1530

This PR fixes it by using cssesc to additionally check if escaping is needed, and not stripping quotes when escaping is needed.

This PR adds cssesc as a dependency, but since it is already a dependency of postcss-selector-parser, it is not effectively adding a new dependency.

[ludofischer]

Thank you! There's a cssnano OpenCollective where you could try to claim some money for your contributions, although unfortunately there's only a few hundred dollars available per year.

[ota-meshi]

I'm grateful that cssnano is continuously maintained! I don't require payment from cssnano 😉