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pref: rework unesc for a 63+% performance boost (#239)

* In profiling postcss I found that a significant amount of time was being
spent in [`unesc`](https://github.com/postcss/postcss-selector-parser/commits/master/src/util/unesc.js), this was due to the expensive regex checks that were
being performed on the fly for every selector in the codebase which looked to be performing quite poorly inside of modern node and v8.

![image](https://user-images.githubusercontent.com/883126/114136698-fdd98a80-98bf-11eb-8068-ace4f6f2274d.png)

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As an experiment and based on some prior experience with this class of slowdown I migrated the implementation to one that performs a scan through the string instead of running a regex replace. By testing this on my local application I instantly saw the work from this function go from > 900 ms to ~100ms.

![image](https://user-images.githubusercontent.com/883126/114136734-0c27a680-98c0-11eb-82ab-f0c9529fd32d.png)

This implementation passes all of the existing test cases and aims to mirror the prior implementation's implementation details :)

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Based on my application I am seeing the major wins come from purgecss dropping my total application build by multiple seconds! 🔥

* Expand unesc handling to correctly handle spec edgecase for lone
surrogates and out of bound codepoint values.

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