Investigate using clangd's vscode client as a dependency.

[dschaefer]

Currently the upstream clangd's vscode extension has more features than what we have put together. If it's actively maintained, we should reuse it.

We may need to update APIs to it to meet our requirements. Our extension will manage the default clangd instance for the user. They can always override, but we'll have one for download and need to be able to point the extension at it.

We also will be managing the compile_commands.json files for various build systems and non-clangd (i.e. gcc cross) toolchains. That may require rekicking clangd when they change.

Of course all of this is candidate for the upstream extension too. But we need to understand what that means.