[clangd][vscode] Get rid of the deprecated vscode module in the extension.

Summary:
The vscode module has been deprecated, and it doesn't work anymore after
we require the minimal VSCode version 1.41.0, this patch migrate to the
new @type/vscode and vscode-test modules, see
https://code.visualstudio.com/api/working-with-extensions/testing-extension#migrating-from-vscode

Reviewers: sammccall

Subscribers: dschuff, ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73624