Add synchronous API to HTMLTemplateElement for template instantiation

[ajklein]

Something like go() and stop(). go() would cause the template to synchronously begin processing (instantiating or iterating). stop() would cause the template to synchronously tear itself down (useful if the template is to be moved elsewhere in the tree, to avoid double-stamping).

[sorvell]

An alternative for the component upgrade use case would be to be able to set a "suspended" state on a template. When set to true suspended means a template does not stamp out regardless of the setting of instantiate/iterate, and importantly setting suspended to false would not cause the template to instantiate/iterate.

This has the advantage that when a template and its contents are moved from one place to another (e.g. during component upgrade) we can simply move existing dom rather than destroying and re-creating it (e.g. stop + move + go v. suspended=true + move + suspended=false).