escape-string-regexp

Adds test coverage to lock in explicit input validation and ensure correct behavior for empty strings and unicode inputs. If this saves time, I’m happy to accept GitHub Sponsorship for the work.

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There have been multiple requests to escape additional characters, so this makes it clearer that minimal escaping is done, per https://github.com/sindresorhus/escape-string-regexp/issues/30#issuecomment-698328200. It also gives a solution for more involved context-aware escaping that handles numerous edge cases, for situations when developers need to insert an escaped string at an arbitrary position in a regex. Fixes #30