Consider adding `is.shiny.tag()` function

It's fairly common for me, I'm not sure about others, to want to know whether or not a particular object is a shiny tag. This is usually done as error checking arguments in a function.

I often add the following function to my utils file:

```
is.shiny.tag <- function(x) {
  inherits(x, "shiny.tag")
}
```

Would you accept a PR to add this to {htmltools}, or would you prefer not?