This change modifies gen_lsp.lua so alias types are generated for
various types of lsp methods to distinguish between notifications
and requests:
- vim.lsp.protocol.Method.ServerToClient.Request
- vim.lsp.protocol.Method.ServerToClient.Notification
- vim.lsp.protocol.Method.ClientToServer.Request
- vim.lsp.protocol.Method.ClientToServer.Notification
These types are then used instead of `string` where appropriate.