Creates a reactive variable that is set to the return value of the function.

The value will be updated when any of the reactive variables used in the function
change.

Example:
```solara
import solara
import solara.lab

a = solara.reactive(1)
b = solara.reactive(2)

@solara.lab.computed
def total():
    return a.value + b.value

def reset():
    a.value = 1
    b.value = 2

@solara.component
def Page():
    print(a, b, total)
    solara.IntSlider("a", value=a)
    solara.IntSlider("b", value=b)
    solara.Text(f"a + b = {total.value}")
    solara.Button("reset", on_click=reset)
```

z.value will be lazily executed the first time, and will be updated
when one of the dependencies changes.