impl Dispose for Callback types and add try_run to the Callable trait

[basro]

PR aims to make the Callback types more consistent with the other stored types in leptos.

• Adds try_run to callable which (in line with other stored types) will return None if the callback had been disposed.
• Adds impl Dispose for both Callback and UnsyncCallback

Small detail regarding implementation:
The impl of Callable::run for UnsyncCallback and Callback was inconsistent. One used try_with_value with expect while the other was using with_value.
I chose to make them consistent and use with_value in both impls. I'm unsure if I chose the right option, perhaps try_with_value was preferable.

[gbj]

I'm very open to adding this.

Note that as it stands, it is a breaking change (adds a required method to a pub trait) so can't be merged until 0.8. I'm currently planning on releasing a 0.8 some time in January that includes any very minor-but-breaking changes like this that have turned up as people test out 0.7 further.