[SSR] SSR v2 scoped slots should handle variable names like generateMarkup

[nolanlawson]

As of #4767 we implemented scoped slots, but we are not correctly handling the case where the bound variable name is already reserved for the framework, e.g. tmpl or generateMarkup. We should do proper shadowing so that these variable names don't conflict.

In other words: component authors shouldn't be able to use variable names like tmpl that change what the framework intends to do.