regex.asciidoc: rephrasing, style, consistency

* Polish some grammar in places.
* Correct some capitalization nitpicks.
* Use "newline" rather than "line feed", which tends to be more common
  in Kakoune's documentation thusfar.

I rephrased some sections, as some of them read a little odd.
* Zero width assertions
    * Consistently use "subject's beginning" instead of "subject begin",
      it reads better.
    * Improve the flow of the word boundary descriptions.
* Modifiers
    * Improve phrasing to emphasize the linear nature of their usage and
      remove a double negative.
    * Use `.` instead of "dot", since that aids in searching through the
      page for things talking about the dot character.
* Compatibility
    * Use asciidoc syntax for the link to the ECMA-262 standard.
    * Use better punctuation on the point about escapes.

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