Check for network connection

[robb-j]

On startup, check for a network connection before running npm install, if a version of the server is already installed, use that.

Alternatively, it could only run the install if the installed version is different from the one in the package.json. e.g.

• .../robb-j.yaml/package.json
• .../robb-j.yaml/node_modules/yaml-language-server/package.json

It might also be worth adding a Extensions → YAML → Reinstall Server command to let users help fix things if they've fiddled around.