extensions: add a default "*" suboptions prefix
This is similar to what we do in other section (e.g. `paths`) and allow to
change the behavior for all extensions.
Sub options on individual extensions overwrite the default settings.
Differential Revision: https://phab.mercurial-scm.org/D11823
{
// 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)
}