Convert method to async should help to change Jetbrain annotations accordingly

[halex2005]

Hi! Thanks for great plugin!

Note that NotNull attribute should be changed to ItemNotNull, and CanBeNull attribute should be changed to ItemCanBeNull.

Before conversion:

[NotNull]
public object MethodName() {
    ...
}
[CanBeNull]
public object MethodName() {
    ...
}

Expected output:

[ItemNotNull]
public Task<object> MethodNameAsync() {
    ...
}
[ItemCanBeNull]
public Task<object> MethodNameAsync() {
    ...
}

For now, attributes are keept unchanged.