Fix caret class on root

[Tidyzq]

regElem({
    is: 'comp',
    options: {
        styleScope: componentSpace.styleScopeManager.register('c'),
    },
    template: '<div id="c" class="c ^c1 ^^c2 ~c3" />',
})
var elem = createElem('comp')

expect(elem.$.c.$$.getAttribute('class')).toBe('c--c c--c1 c--c2 c--c3')
// expected 'c--c c--c1 c--c2 c--c3'
// got 'c--c c1 c--c2 c--c3'

Fixed the issue above, which got wrong style scope when the numbers of ^ exactly equals the depth in shadow tree.

Also refactored the constructor of ClassList for decoupling.