Add option to enable/disable clangd

[ryanplusplus]

I know that this could be done by disabling the extension for a given workspace, but that requires everyone working in the repository to do that via the VS Code GUI. This allows the extension to be disabled in the workspace settings which can be committed to the repo.

In our organization we're looking to transition from using cpptools to clangd and having an easy way to have both extensions installed but only one enabled in a given workspace will be extremely helpful for us. We can already have a workspace that has been migrated to clangd disable Intellisense via cpptools, but there's no equivalent on the clangd side that I was able to find. For workspaces that haven't been migrated, clangd and cpptools will conflict until the clangd extension is disabled manually.