Set options lazily and conditionally

[lionel-]

Addresses posit-dev/positron#8450

Set our options after the user R profiles have been sourced, and only if the options haven't been set by the user already. We detect user overrides in one of two ways:

• If R sets the option to a non-null value by default, we require the user to wrap the option in AsIs, which we take as a sign it was overridden in the Rprofile.

• For all other options, if we see a non-null value it means it was overridden.

I've taken the opportunity to consolidate all options settings in one place (the lazily invoked init function), and document our overrides in doc/configuration.md.

To launch Shiny applications in the system browser you need the following configuration:

options(
  browser = I(Sys.getenv("R_BROWSER")),
  shiny.launch.browser = TRUE
)

An alternative to this approach is to recommend using the new hook that @jennybc is implementing in #1088. The approach implemented in this PR has the advantage that the user configuration also works in the R cli without any duplication.