Add layout_sidebar() to express to make inline sidebar layouts possible

[cpsievert]

The current design of shiny.express.layout.sidebar() doesn't allow for inline sidebar layouts. This would make it a possibility with:

from shiny import ui
from shiny.express import layout

with layout.layout_sidebar():
    ui.sidebar(
        ui.div("Thing 1"), 
        ui.div("Thing 2")
    )
    ui.div("Thing 3")

TODO

Consider the direction proposed by @schloerke #847 (comment) (which would remove the need to have a layout_sidebar(), and would seemingly also allow for inline layouts). I'm not sure we currently have a technical foundation to easily accommodate for this though

[cpsievert]

Closing since layout_sidebar() was added somewhat recently