Improve completions in Service Declaration node context

[malinthar]

Description:

http:Client cli = check new http:Client("");
websub:Listener websubLi = check new websub:Listener(9090);
listener http:Listener httpListener = check new http:Listener(9091);

service on <cursor>  {
    
}

Following improvements can be done in the depicted cursor position

1. The listener declaration httpListener should be sorted to the top
2. We don't suggest visible symbols at the cursor position expect for the listener declarations. We should suggest them even if the are not assignable in the context. For example the websubLi is not a listener declaration. But we can use the following code action to convert it to a listener declaration after providing the completion item.

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