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.

Both are very similar, but Singleton will never re-compute,
both are not settable (read-only).

Example:
```python
from solara.toestand import Computed, Reactive

x = Reactive(1)
y = Reactive(2)

def conditional_add():
    if x.value == 0:
        return 42
    else:
        return x.value + y.value

z = Computed(conditional_add)
```

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

use_task can replace use_thread, but also support coroutine functions.

task is a decorator that creates a top level task that can be executed
from any location (e.g. a click handler of a button).

Co-authored-by: Iisakki Rotko <[email protected]>

Co-authored-by: Iisakki Rotko <[email protected]>

Co-authored-by: Iisakki Rotko <[email protected]>

… not previously set