I don’t know the official way, but one way if you just want to test immediately (obviously highly fragile and hacky) is to update the relevant code in the LanguageServer package code bundled in the VS Code extension. On my system it’s at
~/.vscode/extensions/julialang.language-julia-1.38.2/scripts/packages/LanguageServer
From what I can tell it’s identical as is to the master branch on GitHub.