Implement itemDefaults for CompletionList

[rgrunber]

This depends on #2348 (LSP >= 3.17)

When returning a completion list, almost every time with a text edit where there isn't any element to replace (around the cursor), the entry looks like this :

"textEdit": {
    "newText": "getenv()",
    "insert": {
        "start": {
            "line": 36,
            "character": 15
        },
        "end": {
            "line": 36,
            "character": 18
        }
    },
    "replace": {
        "start": {
            "line": 36,
            "character": 15
        },
        "end": {
            "line": 36,
            "character": 18
        }
    }
}

Insert and replace are identical. It would be nice to de-deduplicate this. There might also be other values within completion item that we might be able to set as a default.

https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#completionList

       /**
	 * In many cases the items of an actual completion result share the same
	 * value for properties like `commitCharacters` or the range of a text
	 * edit. A completion list can therefore define item defaults which will
	 * be used if a completion item itself doesn't specify the value.
	 *
	 * If a completion list specifies a default value and a completion item
	 * also specifies a corresponding value the one from the item is used.
	 *
	 * Servers are only allowed to return default values if the client
	 * signals support for this via the `completionList.itemDefaults`
	 * capability.
	 *
	 * @since 3.17.0
	 */
         itemDefaults?: {