Add support for fern.vim (andweeb#86)

* feat(file_explorers): added support for `fern.vim`
* fix: missing trailing comma

see https://github.com/lambdalisue/fern.vim

Author: wuelnerdotexe

lua/presence/file_explorers.lua

@@ -5,5 +5,6 @@ return {
     ["[defx] default-"] = "Defx",
     ["netrw"] = "Netrw",
     ["TelescopePrompt"] = "Telescope",
-    ['neo-tree'] = 'Neotree'
+    ['neo-tree'] = 'Neotree',
+    ['fern'] = 'Fern'
 }