Rename suspend_display->hold, display_body->expressify, render.display-> render.express

[wch]

This PR supersedes #996. It does the following:

• Rename express.suspend_display -> express.ui.hold
• Rename express.display_body -> express.expressify
• Rename render.display -> render.express

Also, for hold:

• hold() now can only be used a context manager, and not as a decorator.
• You can do with ui.hold() as x, and then use x later on the page to display it.

Note that suspend_display is basically an alias for hold, and in this PR it has lost the ability to be used as a decorator, which could break code that was using the previous (experimental) API.

Example usage for hold:

import shiny.ui
from shiny.express import input, render, ui

# `ui.hold() as x` can be used to save `x` for later output
with ui.hold() as hello_card:
    with ui.card():
        with ui.span():
            "This is a"
            ui.span(" card", style="color: red;")

hello_card
hello_card


# ================================================

# `ui.hold()` can be used to just suppress output
with ui.hold():
    @render.text()
    def txt():
        return f"Slider value: {input.n()}"


ui.input_slider("n", "N", 1, 100, 50)
shiny.ui.output_text("txt", inline=True)

Example for expressify:

from shiny.express import expressify, ui

# @expressify converts a function to work with Express syntax
@expressify
def expressified1(s: str):
    f"Expressified function 1: {s}"
    ui.br()


expressified1("Hello")
expressified1("world")

Example for render.express:

from shiny.express import input, render, ui

ui.input_slider("n", "N", 1, 100, 50)

# @render.express is like @render.ui, but with Express syntax
@render.express
def render_express1():
    "Slider value:"
    input.n()

[jcheng5]

No docstrings for this yet? (Or do they live somewhere else?)