Update MD037/no-space-in-emphasis to handle multiple embedded underscores separated by a single character (fixes #804).

test/snapshots/markdownlint-test-scenarios.js.md

@@ -43625,6 +43625,9 @@ Generated by [AVA](https://avajs.dev).
       Emphasis <img alt="inside * attribute * content"/> {MD033}␊
       ␊
       Emphasis <p data="* attribute *">*HTML*</p> {MD033} {MD037}␊
+      ␊
+      Embedded underscore is okay:␊
+      Text _emphas_i_s_ text _emphasis_␊
       `,
     }