Support native VS Code citation picker

[jooyoungseo]

Currently, this extension only uses Zotero's quick citation picker. However, it would be even nicer if users could switch to native VS Code citation picker, which could be much more accessible and seamless. FYI, the native citation picker works better for screen readers and other assistive technologies.

For example, tecosaur/LaTeX-Utilities#29 allows this feature:

{
    "latex-utilities.zotero.citeMethod": "vscode"
}

However, that extension currently do not support Pandoc style citation.