Move formatting to server side

[antico5]

Right now we are executing code formatting (prettier/forge fmt) on the client side. We could move that to the server side by implementing the textDocument/willSaveWaitUntil method, or the textDocument/formatting method (probably VSCode issues the latter when the "format on save" setting is enabled)

[kanej]

Our support for forge fmt is run on the command line. It takes formatting settings from the context it is executed in.

The PR #316 changes the context to run forge fmt in the root of the workspace folder.

On moving formatting to server side we would have the option to leverage project information to run the formatting command in the folder where the forge.toml file exists, supporting formatting rules across a monorepo for instance.

[kanej]

vim.coc supports the format command. So if we add it to the language server, vim gets it as well.