(#14093) Remove unsafe attributes from TemplateWrapper

The attribute accessor for the :string attribute in the TemplateWrapper
conflicted with a variable being in scope that is named string. The
variable would take precedence and because of the way the TemplateWrapper
accessed the value of the attribute, the variable's value would be used as
the template.

This changes the TemplateWrapper to completely remove :string as an
attribute and only use local variables. This also removes the :file
attribute in favor of a "private"-style naming of the instance variable.
There are no collisions with @__file__ that would cause problems, but this
should reduce the likelihood of them happening in the first place.