The purpose is to have a clearer view of placeholders used in a
template, without going the usual `render(Template, #{})` route that is
actually subtly misleading: it won't mention that `${}` / `${.}`
placeholder has been used.
Also unify handling of `${}` / `${.}` in a couple of places.
Even though most of the time these modules will be used by
connectors, there are exceptions (namely, `emqx_rule_engine`).
Besides, they are general enough to land there, more so given
that `emqx_placeholder` is already there.
Andrew Mayorov