Allow send actions that are conditionally executed (#1303)

New semantics for worker message passing, which allows send actions to be conditionally executed.
Most of the _Worker message passing_ section is rewritten.
Fixes #1273.

lang/lib/error.bal

@@ -83,6 +83,9 @@ public isolated function toString(error e) returns string = external;
 # defined in the Ballerina Language Specification, using the expression style.
 public isolated function toBalString(error e) returns string = external;
 
+# The type of error for a receive-action when no message has been sent.
+public type NoMessage distinct error;
+
 # A type of error which can be retried.
 public type Retriable distinct error;