Value.get: Accept "default" parameter

[pfalcon]

The idea is similar to e.g. Python's dict.get() method, which can take an optional default if a given key doesn't exist in the dict.

This change would allow to users to "bootstrap" otherwise circular dependencies in simple cases. For example, it allows to turn this code:

from shiny.express import input, render, ui

@render.express
def myrender():
    ui.input_select("sel", None, ["All", "So", "Much"], selected="All")
    input.sel()

which doesn't render anything nor produces errors (underneath it throws SilentException which isn't reported) into following code:

from shiny.express import input, render, ui

@render.express
def myrender():
    ui.input_select("sel", None, ["All", "So", "Much"], selected=input.sel.get("All"))
    input.sel.get("All")

which works as expected (initial selected value is "All", and if user chooses another, it's preserved across myrender() run).