Make page_fillable() the default for express mode

[cpsievert]

Closes #846.

With this PR, page_fillable() becomes the new page default for express. This should lead to the same experience as before for non-fill items (e.g., inputs, text, etc), but fill items like plots, cards, layouts, etc. will now fill the page (when wide enough).

Moreover, this default is more consistent with the with layout.sidebar() experience, which treats the main content area as a fillable container (by default).

For a concrete example, notice that the plot here will now fill the page:

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

ui.input_slider("slider", label="Slider", min=0, max=100, value=50)

@render.plot()
def plt():
    import matplotlib.pyplot as plt
    import numpy as np
    x = np.linspace(0, 2 * np.pi, 100)
    y = np.sin(x * input.slider())
    fig, ax = plt.subplots()
    ax.plot(x, y)
    return fig

[cpsievert]

I decided that it'd be better to do separate PRs for the issues we ran into earlier today (see #859 and #861) since they're orthogonal to this change. I also sent a PR for the doc change you requested wch/shiny_express_doc#1

So, feel free to merge this as-is when you're ready.