Add autocompletion support in `.swift-format` files

**Is your feature request related to a problem? Please describe.** The VSCode extension provides autocompletion support for `Package.swift`. The Swift toolchain comes with `swift-format`, who configuration file `.swift-format` is a JSON file. It would be create if the extension supported autocompletion in the `.swift-format`, similar to how VSCode has autocompletion in when configuration the IDE settings in JSON.