Add render.download, take 2

[wch]

This PR adds render.download, and supersedes #967.

The existing API is to use session.download, like this:

# UI contains:
ui.download_button("dl", "Download File")

# Server function contains:
@session.download(filename="hello.txt")
def dl():
    yield "Hello, world!\n"

This is quite a bit different from all other outputs, because it relies on the session object. This also doesn't work for Express apps, because there isn't a session object available. (I'll note here that it would be possible to add a session object that could be imported from shiny.express, but the change in this PR is a better one.)

With this PR, the app uses render.download instead of session.download:

# UI contains:
ui.download_button("dl", "Download File")

# Server function contains:
@render.download(filename="hello.txt")
def dl():
    yield "Hello, world!\n"

I think this makes more sense from the app author's perspective.

In an Express app, the code would look like this (the button is automatically inserted in the UI):

@render.download(filename="hello.txt")
def dl():
    yield "Hello, world!\n"

With Express, the label can also be set with label:

@render.download(filename="hello.txt", label="Download!")
def dl():
    yield "Hello, world!\n"

Currently, label is the only parameter for download_button that's exposed in render.download. There are others, like icon and class_ which are relatively commonly used -- maybe it's best to wait until we resolve #971 before we decide what to do with these other parameters.

[schloerke]

LGTM! (given cosmetic tweaks)

[wch]

Going to hold the output and Session removal for later, since those are already covered by #793.