[Spec Deviation] Support declaring listeners without the type-descriptor

[MaryamZi]

Description:
According to the spec

listener-decl :=
   metadata
   [public] listener [type-descriptor] identifier = expression ;

A listener-decl defines a module listener.

A module-listener can be referenced by a variable-reference, but cannot be modified. It is this similar to a final variable declaration, except that it also registers the value with the module as a listener.

A module listener has a static type, which must be a subtype of the Listener type. If the type-descriptor is present it specifies the module listener's static type; if it is not present, the the static type of the listener is the static type of expression.

Steps to reproduce:
But the current implementation does not support declaring listeners without the type-descriptor.

import ballerina/http;

listener xyz = new http:Listener(8080);

$ ballerina run test.bal 
error: .::test.bal:3:14: mismatched input '='. expecting {'[', '?', '|', Identifier}
error: .::test.bal:3:24: mismatched input ':'. expecting '='
error: .::test.bal:3:33: mismatched input '('. expecting {'[', '?', '|', Identifier}

Affected Versions:

$ ballerina -v
Ballerina 1.0.0-alpha3
Language specification 2019R2
Ballerina tool 1.0.0