fix(api): nvim_open_tabpage positional "enter"

Problem: nvim_open_tabpage's "enter" argument is optional, which is inconsistent
with nvim_open_win.

Solution: make it a (non-optional) positional argument, like nvim_open_win.

Also change "enter"'s description to be more like nvim_open_win's doc.

runtime/lua/vim/_meta/api.lua

@@ -1677,12 +1677,12 @@ function vim.api.nvim_notify(msg, log_level, opts) end
 ---
 --- @param buffer integer Buffer to open in the first window of the new tabpage.
 --- Use 0 for current buffer.
+--- @param enter boolean Enter the tabpage (make it the current tabpage).
 --- @param config vim.api.keyset.tabpage_config Configuration for the new tabpage. Keys:
---- - enter: Whether to enter the new tabpage (default: true)
 --- - after: Position to insert tabpage (default: -1; after current).
 ---          0 = first, N = after Nth.
 --- @return integer # Tabpage handle of the created tabpage
-function vim.api.nvim_open_tabpage(buffer, config) end
+function vim.api.nvim_open_tabpage(buffer, enter, config) end
 
 --- Open a terminal instance in a buffer
 ---

runtime/lua/vim/_meta/api_keysets.lua

@@ -438,7 +438,6 @@ error('Cannot require a meta file')
 --- @field _subpriority? integer
 
 --- @class vim.api.keyset.tabpage_config
---- @field enter? boolean
 --- @field after? integer
 
 --- @class vim.api.keyset.user_command