Include Default Parameter Values in Signature Help

From https://github.com/Microsoft/vscode/issues/28925

TypeScript Version: 2.4.0

Feature Request
signatureHelp current shows when a parameter is optional but does not include any information about that parameter's default value:

function foo(x = 10) { }
foo(|)

Result of TSServer signatureHelp on foo

[Trace  - 5:42:22 PM] Response received: signatureHelp (378). Request took 20 ms. Success: true 
Result: {
    "items": [
        {
            "isVariadic": false,
            "prefixDisplayParts": [
                {
                    "text": "foo",
                    "kind": "functionName"
                },
                {
                    "text": "(",
                    "kind": "punctuation"
                }
            ],
            "suffixDisplayParts": [
                {
                    "text": ")",
                    "kind": "punctuation"
                },
                {
                    "text": ":",
                    "kind": "punctuation"
                },
                {
                    "text": " ",
                    "kind": "space"
                },
                {
                    "text": "void",
                    "kind": "keyword"
                }
            ],
            "separatorDisplayParts": [
                {
                    "text": ",",
                    "kind": "punctuation"
                },
                {
                    "text": " ",
                    "kind": "space"
                }
            ],
            "parameters": [
                {
                    "name": "x",
                    "documentation": [],
                    "displayParts": [
                        {
                            "text": "x",
                            "kind": "parameterName"
                        },
                        {
                            "text": "?",
                            "kind": "punctuation"
                        },
                        {
                            "text": ":",
                            "kind": "punctuation"
                        },
                        {
                            "text": " ",
                            "kind": "space"
                        },
                        {
                            "text": "number",
                            "kind": "keyword"
                        }
                    ],
                    "isOptional": true
                }
            ],
            "documentation": [],
            "tags": []
        }
    ],
    "applicableSpan": {
        "start": {
            "line": 3,
            "offset": 5
        },
        "end": {
            "line": 3,
            "offset": 5
        }
    },
    "selectedItemIndex": 0,
    "argumentIndex": 0,
    "argumentCount": 0
}

When the default value is a simple literal type, it would be helpful to display this default value in the signature help. This information could be included in the displayParts response

issue に示されている TSServer signatureHelp レスポンスから始め、TypeScript 言語サービスのシグネチャヘルプ実装を追跡します。パラメーターの displayParts がどのように組み立てられるかを確認し、そのレスポンスに単純なデフォルト値のケースが現れるようにして、foo の例に対して結果のシグネチャヘルプを検証します。