Highlight backslash escapes within $'' strings.

Correct highlighting of backslash escapes within "" strings: highlight only
the four specific escape sequences defined there.

Fixes zsh-users/zsh-syntax-highlighting#196.

highlighters/main/README.md

@@ -45,6 +45,7 @@ This highlighter defines the following styles:
 * `dollar-quoted-argument` - dollar quoted arguments (`` $'foo' ``)
 * `dollar-double-quoted-argument` -  dollar double quoted arguments ($foo inside "")
 * `back-double-quoted-argument` -  back double quoted arguments (\x inside "")
+* `back-dollar-quoted-argument` -  back dollar quoted arguments (\x inside $'')
 * `assign` - variable assignments
 * `default` - parts of the buffer that do not match anything