feat(client): add Vim split/tab/back link-follow commands + true tab-drop

The plain-Vim client only defined VimwikiFollowLink; the split/vsplit/
tabnew/tabdrop/back command surface existed only in the Neovim branch.
Add :Vimwiki{Split,VSplit,Tabnew,TabDrop,GoBack}Link (+ :Nuwiki* aliases)
to the Vim branch, backed by a new nuwiki#commands#follow_link_drop()
helper that runs `:tab drop` to reuse an already-open tab.

Both clients now implement real tab-drop: open_uri gains a 'tabdrop'
case and M.follow_link_drop(); the Neovim TabDropLink commands and the
<C-S-CR> mappings (Vim + Lua) are repointed off the old tabnew cheat.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

autoload/nuwiki/commands.vim

@@ -188,6 +188,47 @@ function! s:badd_from_response(notification) abort
   echom 'nuwiki: added ' . l:path
 endfunction
 
+" `:VimwikiTabDropLink` — open the link target in a tab, reusing an
+" existing tab/window if the file is already shown (`:tab drop`). Routes
+" through vim-lsp's textDocument/definition like `badd_link`.
+function! nuwiki#commands#follow_link_drop() abort
+  if s:has_vimlsp()
+    call lsp#send_request(s:server, {
+          \ 'method': 'textDocument/definition',
+          \ 'params': s:cursor_position(),
+          \ 'on_notification': function('s:drop_from_response'),
+          \ })
+    return
+  endif
+  if s:has_coc()
+    let l:locs = CocAction('getDefinition')
+    if type(l:locs) == type([]) && !empty(l:locs)
+      let l:uri = get(l:locs[0], 'uri', get(l:locs[0], 'targetUri', ''))
+      if !empty(l:uri)
+        let l:path = substitute(l:uri, '^file://', '', '')
+        execute 'tab drop ' . fnameescape(l:path)
+      endif
+    endif
+    return
+  endif
+  echohl ErrorMsg | echom 'nuwiki: no supported LSP client for follow_link_drop' | echohl None
+endfunction
+
+function! s:drop_from_response(notification) abort
+  let l:result = get(get(a:notification, 'response', {}), 'result', {})
+  let l:uri = ''
+  if type(l:result) == type({})
+    let l:uri = get(l:result, 'uri', get(l:result, 'targetUri', ''))
+  elseif type(l:result) == type([]) && !empty(l:result)
+    let l:uri = get(l:result[0], 'uri', get(l:result[0], 'targetUri', ''))
+  endif
+  if l:uri ==# ''
+    return
+  endif
+  let l:path = substitute(l:uri, '^file://', '', '')
+  execute 'tab drop ' . fnameescape(l:path)
+endfunction
+
 " ===== Smart <CR>: follow or wrap-then-follow =====
 
 function! s:cursor_inside_wikilink() abort