Add render.code()

[wch]

This PR adds a function render.code(), and, in Shiny Express, changes the output behavior for render.text().

In Shiny Express:

• render.text() now uses output_text() as its output function, instead of output_text_verbatim().
• The new render.code() uses output_text_verbatim() as its output function.

In Shiny Core:

• render.text() and render.code() do the same thing, although the documentation now suggests pairing render.text with output_text(), and render.code() with output_text_verbatim().

In the future, when we add output_code() (perhaps with line numbers and syntax highlighting), we can change render.code() to use that output function instead of output_text_verbatim().

This is a screenshot of usage in a Shiny Express app:

[wch]

The changes here were incorporated in #1018.