[TEXT-XXX] Support ECMAScript \x unescaping in StringEscapeUtils

[timdp]

Currently, StringEscapeUtils unescapes ECMAScript using the Java strategy.

This mostly works, but ECMAScript additionally supports \xCC escapes, where CC is a hex code.

This PR adds support for that, as well as an initial test.

I took the liberty of making the child translators of the Java aggregate translator reusable, so I could basically extend the list for the ES version. If we prefer to duplicate that code or to use a varargs list or something, I'm absolutely fine with that as well.

I just subscribed to the dev list and requested access to Jira. I will update the PR title once I've created the corresponding issue.