Should have version of `isolate()` which takes a function as an argument

The existing with isolate(): syntax is good for running blocks of code, but it is not convenient when you simply want to get a value and use it in line. For example, suppose we want to get the value of `input.x

@output
@render.text
def txt():
    with isolate():
        capitalize = input.capitalize()
    if capitalize:
        return input.txt().upper()
    else:
        return input.txt()

In many cases, we just want to fetch a reactive value (or reactive Calc) inline, but the with isolate(): syntax makes that clunky and requires an extra variable to store the result.

It would be nice to be able to simply pass a function to isolate(), or perhaps a different function with a name like run_isolated().

For example:

@output
@render.text
def txt():
    if run_isolated(input.capitalize):
        return input.txt().upper()
    else:
        return input.txt()

Start by locating the existing isolate() implementation and its surrounding reactive entry points. Compare the requested callable form with the current with isolate(): behavior, then determine the API and verification needed for inline isolated reads to return the callable's value without creating a temporary variable.