Dev mode aware client and duplicate input/output ID handling updates

[nstrayer]

This is a follow-up to #3946 and a fix for #3950.

The main contribution is the passing of dev mode status to the client. This is done by injecting a script tag into the head of the app when in dev mode. This script tag adds a global variable to the window that is then used as a signal to the client that dev mode is activated. To avoid awkward global-variable-consuming we provide a helper method on the Shiny object: Shiny.inDevMode() that abstracts this away.

This method for relaying dev mode status is inspired by how React does it.

The ability for the client to know about dev mode status is utilized to more explicitly label the passthrough case of duplicated input and output IDs in the code...

// If there's a single duplication of ids across both binding types, then
// when we're not in devmode, we allow this to pass because a good amount of
// existing applications use this pattern even though its invalid. Eventually
// this behavior should be removed.
if (counts.input === 1 && counts.output === 1 && !Shiny.inDevMode()) {
  return;
}

// If we have duplicated IDs, then add them to the set of duplicated IDs
// to be reported to the user.
if (counts.input > 0 || counts.output > 0) {
  duplicateIds.set(id, counts);
}

... while also flagging this as an error when in dev mode.