feat(api): add nvim_open_tabpage

Problem: no API function for opening a new tab page and returning its handle, or
to open without entering.

Solution: add nvim_open_tabpage.

runtime/lua/vim/_meta/api_keysets.lua

@@ -437,6 +437,10 @@ error('Cannot require a meta file')
 --- @field scoped? boolean
 --- @field _subpriority? integer
 
+--- @class vim.api.keyset.tabpage_config
+--- @field enter? boolean
+--- @field after? integer
+
 --- @class vim.api.keyset.user_command
 --- @field addr? any
 --- @field bang? boolean