fix: prevent NvimTree to be alternate buffer when tab open

[phanen]

<c-t> on file, then sometimes I press <c-^> and mistakenly goto NvimTree buffer.

[alex-courtis]

Tested OK for base below and a few variants:

1. :NvimTreeOpen
2. <navigate to file>
3. <C-t>
4. <C-^> or :e# or :b#

:ls!
  1  a   "[No Name]"                    line 1
  2u a-  "NvimTree_1"                   line 0
  3 %a   ".clang-format"                line 1

No # on nvim tree buffer ✔️

[alex-courtis]

Many thanks for your contribution!

Looks good, works well, just a few questions.

[alex-courtis]

Really appreciate all your work @phanen