evaluate generic server-side snippets in resolve stage

[Eskibear]

See #1838 , perf issue about evaluating generic snippets.

There are two groups of Snippets:

• Generic snippets (e.g. foreach, fori, ifelse, etc.)
• Type definition snippets (e.g. class, interface, etc.)

Currently at completion stage, we evaluate template patterns in given context to provide insertText and documentation. At resolve stage we do nothing.

Evaluation is relatively expensive. This PR does below things:

• For generic snippets, delay resolving evaluation, providing corresponding TextEdit and documentation at resolve stage. In insertText, placeholders are directly from template patterns.
• For type definition snippets, remove unused data field as they are not processed in at resolve stage.

[Eskibear]

/**
* An edit which is applied to a document when selecting this completion.
* When an edit is provided the value of insertText is ignored.
.....
textEdit?: TextEdit | InsertReplaceEdit;

It's implemented based on LSP specs above. After "completion", insertText is effective to make sure the snippet structure is correct. After being resolved, textEdit becomes effective, and all the placeholders are filled by corresponding variable names.

Below is a sample response for foreach snippet:

public class App
{
    public static void main( String[] args )
    {
        foreach^
    }
}

Trace - 上午12:38:58] Received response 'textDocument/completion - (80)' in 17ms.
Result: {
    "isIncomplete": false,
    "items": [
        {
            "label": "foreach",
            "kind": 15,
            "detail": "iterate over an array or Iterable",
            "insertText": "for (${1:iterable_type} ${2:iterable_element} : ${3:iterable}) {\n\t$TM_SELECTED_TEXT${0}\n}",
            "insertTextFormat": 2,
            "data": {
                "rid": "5",
                "uri": "file:///C:/Users/admin/Desktop/demo/src/main/java/com/example/App.java",
                "pid": "0"
            }
        }
    ]
}

[Trace - 上午12:38:58] Received response 'completionItem/resolve - (82)' in 6ms.
Result: {
    "label": "foreach",
    "kind": 15,
    "detail": "iterate over an array or Iterable",
    "documentation": {
        "kind": "markdown",
        "value": "```java\nfor (String string : args) {\n\t\n}\n```"
    },
    "insertText": "for (${1:iterable_type} ${2:iterable_element} : ${3:iterable}) {\n\t$TM_SELECTED_TEXT${0}\n}",
    "insertTextFormat": 2,
    "textEdit": {
        "range": {
            "start": {
                "line": 10,
                "character": 15
            },
            "end": {
                "line": 10,
                "character": 15
            }
        },
        "newText": "for (${1:String} ${2:string} : ${3:args}) {\n\t$TM_SELECTED_TEXT${0}\n}"
    }
}

[Eskibear]

About the perf boost, I simply tested spring-petclinic project on a Macbook Pro(2016), 8G-RAM, 2.9GHz Dual-Core i5.
Left side with light theme is v0.81.0, right side with dark theme the LS has included this PR.

By typing an f (with several snippet candidates like fori, foreach), triggering completion for 3 times, the avg time cost is reduced from ~190ms to ~80ms.

For a single completion of foreach snippet, now most time cost is moved to resolve stage.

[rgrunber]

Works for me. I can confirm the resolution is now done in resolveCompletionItem. Just some minor changes to make.

[rgrunber]

Change looks good to me.