Get value from bookmark or return default.
restoreValue(value, default)In an application restored from a server-side bookmark,
the variable specified by value from the values environment.
Otherwise default.
(character(1)) name of value to restore
fallback value
Bookmarks can store not only inputs but also arbitrary values.
These values are stored by onBookmark callbacks and restored by onBookmarked callbacks,
and they are placed in the values environment in the session$restoreContext field.
Using teal_data_module makes it impossible to run the callbacks
because the app becomes ready before modules execute and callbacks are registered.
In those cases the stored values can still be recovered from the session object directly.
Note that variable names in the values environment are prefixed with module name space names,
therefore, when using this function in modules, value must be run through the name space function.