fix(lsp): add coc.nvim dispatch path to all Vim-side LSP calls

Commands were hardwired to vim-lsp (lsp#send_request, :LspDefinition,
:LspReferences, :LspRename). Users running coc.nvim got "vim-lsp not
loaded" on every command despite the plugin correctly detecting coc.

Changes:

autoload/nuwiki/commands.vim
- Add s:has_coc() (checks CocActionAsync exists)
- Add s:coc_wrap() adapter — converts coc's (err, result) callback into
  the vim-lsp notification shape {response:{result:…}} so all existing
  on_notification handlers work unchanged
- s:exec(): try vim-lsp first, fall back to CocActionAsync('runCommand',
  …) with arguments spread as positional params, then error if neither
- Add s:jump_definition() helper — :LspDefinition vs CocActionAsync
- Add nuwiki#commands#backlinks() — :LspReferences vs CocActionAsync
- badd_link(): coc path uses CocAction('getDefinition') to resolve the
  target URI without navigating
- follow_link_or_create(): replace inline :LspDefinition checks with
  s:jump_definition()
- rename_file(): :LspRename vs CocActionAsync('rename')

ftplugin/vimwiki.vim
- VimwikiFollowLink / NuwikiFollowLink: dispatch through
  nuwiki#commands#follow_link_or_create() instead of :LspDefinition
- VimwikiBacklinks / NuwikiBacklinks / VWB: dispatch through
  nuwiki#commands#backlinks() instead of :LspReferences

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ftplugin/vimwiki.vim

@@ -39,9 +39,9 @@ if !has('nvim')
   command! -buffer        VimwikiDiaryNextDay           call nuwiki#commands#diary_next()
   command! -buffer        VimwikiDiaryPrevDay           call nuwiki#commands#diary_prev()
   command! -buffer        VimwikiDiaryGenerateLinks     call nuwiki#commands#diary_generate_index()
-  command! -buffer        VimwikiFollowLink             LspDefinition
-  command! -buffer        VimwikiBacklinks              LspReferences
-  command! -buffer        VWB                           LspReferences
+  command! -buffer        VimwikiFollowLink             call nuwiki#commands#follow_link_or_create()
+  command! -buffer        VimwikiBacklinks              call nuwiki#commands#backlinks()
+  command! -buffer        VWB                           call nuwiki#commands#backlinks()
   command! -buffer        VimwikiNextLink               call nuwiki#commands#link_next()
   command! -buffer        VimwikiPrevLink               call nuwiki#commands#link_prev()
   command! -buffer        VimwikiBaddLink               call nuwiki#commands#badd_link()
@@ -88,8 +88,8 @@ if !has('nvim')
   command! -buffer        NuwikiDiaryTomorrow  call nuwiki#commands#diary_tomorrow()
   command! -buffer        NuwikiDiaryNext      call nuwiki#commands#diary_next()
   command! -buffer        NuwikiDiaryPrev      call nuwiki#commands#diary_prev()
-  command! -buffer        NuwikiFollowLink LspDefinition
-  command! -buffer        NuwikiBacklinks  LspReferences
+  command! -buffer        NuwikiFollowLink call nuwiki#commands#follow_link_or_create()
+  command! -buffer        NuwikiBacklinks  call nuwiki#commands#backlinks()
   command! -buffer        NuwikiDeleteFile call nuwiki#commands#delete_file()
   command! -buffer        NuwikiRenameFile call nuwiki#commands#rename_file()
   command! -buffer        NuwikiNextTask           call nuwiki#commands#next_task()