vim: Improve conditions for automatic insert mode on TermOpen

When a terminal buffer is opened (TermOpen), for example, by `:term`,
we would want to automatically enter the insert mode. However,
the existing autocmd may result in the non-terminal buffer unwantedly
entering the insert mode when a new terminal buffer is opened on a
floating window. To prevent this, we can enter the insert mode
only if the current window has the terminal buffer being opened.

See nvim-neotest/neotest#2 for reference.

vim/vimrc

@@ -431,7 +431,7 @@ if has('nvim')
   augroup terminal_autoinsert
     autocmd!
     autocmd BufWinEnter,WinEnter *  if &buftype == 'terminal' | startinsert | endif
-    autocmd TermOpen * startinsert
+    autocmd TermOpen *  if nvim_buf_get_name(0) =~# '^term://' | startinsert | endif
 
     " mouse click puts into normal mode even in terminal; disable this
     autocmd TermOpen * tnoremap <buffer><silent> <LeftRelease> <Nop>