Fix zero-width regex replace all (#775)

[MihneaTeodorStoica]

Summary

• fix find_and_replace_all() so zero-width regex matches advance past the replacement instead of re-matching the same location
• return the matched range from find_select_next() so replace-all can distinguish zero-width matches from normal matches
• add regression tests for both single replace and replace-all with the $ regex

Testing

• cargo test replace_all_supports_zero_width_regex_matches --lib -- --nocapture
• cargo test replace_supports_zero_width_regex_matches --lib -- --nocapture
• cargo test