Expressions should be usable within <script> tag

[clarkevans]

Currently, JSExpr uses JSON to serialize strings. This is problematic since JSON doesn't include \s, \S, \/ and \! as valid escape sequences. Therefore JSExpr should probably not depend upon JSON but instead implement its own String serialization where <script> is escaped as <\script>, among others.

See https://html.spec.whatwg.org/multipage/scripting.html#restrictions-for-contents-of-script-elements for more detail.