Add dev_mode param to App()

[wch]

This PR does a few things:

• It adds a new parameter so you can call App(dev_mode=True).
• When the App object is created, if dev_mode is None (the default), then it looks at the env var SHINY_DEV_MODE. The setting for dev_mode is stored in the App object.
• Before the change in this PR, it only used that env var to determine whether to dev mode, and it looked at the env var each time that shiny_deps() was called. This is potentially problematic when running multiple concurrent apps, as with shinylive apps embedded in a web page. With the change, the App object stores the value of dev_mode, so the setting for one App will not affect any others.

The reason for this PR is for Shinylive -- I want to enable the dev mode error console when there's are both editor and viewer components, but not when there's just the viewer component.