.jshintrc

exthelper: reintroduce the ability to register revsets

I think this is what Yuya and Boris agreed on.[1]  This happens *after* the
extsetup phase now (and after the _aftercallback notifications).  But this is
trivial, mergeable between exthelper instances, and doesn't need to have the
extension name supplied when registering.

The test needed updating so that extsetup() takes a `ui` argument, as exthelper
isn't trying to be backward compatible with 1.3.1.

[1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2018-December/125888.html

{
    // Enforcing
    "eqeqeq"        : true,     // true: Require triple equals (===) for comparison
    "forin"         : true,     // true: Require filtering for..in loops with obj.hasOwnProperty()
    "freeze"        : true,     // true: prohibits overwriting prototypes of native objects such as Array, Date etc.
    "nonbsp"        : true,     // true: Prohibit "non-breaking whitespace" characters.
    "undef"         : true,     // true: Require all non-global variables to be declared (prevents global leaks)

    // Environments
    "browser"       : true      // Web Browser (window, document, etc)
}