vscode: add support for inline completions

[vince-fugnitto]

What it does

Closes: #11868.

The pull-request adds support for the VS Code InlineCompletions related APIs (non-proposed):

• InlineCompletionItem
• InlineCompletionItemProvider
• InlineCompletionList
• InlineCompletionTriggerKind
• InlineCompletionContext
• registerInlineCompletionItemProvider

inline-completions-api.mp4

How to test

1. start the application with the following https://github.com/eclipse-theia/theia/files/10059321/inline-completions-0.0.1.zip
2. open an editor
3. hello world should be proposed as an inline completion

import * as vscode from 'vscode';

export function activate(context: vscode.ExtensionContext) {
    const provider = createInlineCompletionItemProvider();
    vscode.languages.registerInlineCompletionItemProvider({ pattern: '**' }, provider);
}

export function deactivate() { }

function createInlineCompletionItemProvider(): vscode.InlineCompletionItemProvider {
    return {
        provideInlineCompletionItems(document, position, context, token) {
            return [{
                insertText: "hello world",
            }];
        },
    }
}

CQ

• CQ Approved

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

The CQ for the pull-request has been approved 👍

[msujew]

Looks quite good to me. I can confirm that the functionality works as expected with the provided extension and the APIs show as supported in the comparator report.

I have a question regarding naming, see below. Otherwise LGTM 👍

[msujew]

LGTM 👍