Revert Shiny.renderContent() back to sync instead of async

[cpsievert]

Motivated by rstudio/bslib#872

This PR essentially reverts the changes made to srcts/src/shiny/render.ts in #3904, then does:

1. Keeps the shinyBindAll(el) -> await shinyBindAll(el) change made to renderContentAsync()
2. Adds a shinyBindAll(el) -> return shinyBindAll(el) change in renderContent() (so you can await the bind if need be)

[wch]

If we were to use the synchronous code path for loading these resources, then it won't work in shinylive on Chrome (because Chrome's service worker implementation doesn't intercept synchronous XHR requests, which is what jQuery users for synchronous loading of JS resources).

I'll comment on the bslib issue about possible ways forward there.

[wch]

For renderContent, please add some information about why this won't work in shinylive (and about the synchronous XHR warnings even outside of shinylive, #789). Also explain what parts of this function happen synchronously, and what part (the bindAll is async, and how that could be an issue in some cases.

And for renderContentAsync, tell people that this is what they should use going forward.

[cpsievert]

There's a pretty good (and lengthy) comment just above renderContentAsync() on this topic