Add flag to mark TS Server unused variable suggestion diagnostics

[mjbvz]

TypeScript Version: 2.9.0-dev.20180409

Search Terms:

• suggestion diagnostics
• no unused
• tsserver

Request
Follow up on #22361. Just tried it out in VS Code. Very cool to see this working but we will need a new a flag on suggestion diagnostic objects that identifies them as being diagnostics for unused variables. I don't currently see any flags being returned by tsserver

[ghost]

We did add a reportsUnnecessary property to the diagnostic object.

[mjbvz]

Ok, is it possible that this property is not being sent through the tsserver? Here's what I see for an unused diagnostic on the 2.9.0-dev.20180409 insiders build:

    "diagnostics": [
        {
            "start": {
                "line": 3,
                "offset": 7
            },
            "end": {
                "line": 3,
                "offset": 8
            },
            "text": "'a' is declared but its value is never read.",
            "code": 6133,
            "category": "suggestion"
        }
    ]