feat(lsp): use stricter types for methods

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.

src/gen/gen_vimdoc.lua

@@ -427,6 +427,9 @@ end
 --- @param generics? table<string,string>
 --- @param default? string
 local function render_type(ty, generics, default)
+  -- TODO(lewis6991): Document LSP protocol types
+  ty = ty:gsub('vim%.lsp%.protocol%.Method.[%w.]+', 'string')
+
   if generics then
     ty = replace_generics(ty, generics)
   end