feat: add njk -> jinja filetype mapping

- Created `core/filetypes.lua` to map `.njk` files to the `jinja` filetype
- Updated `core/init.lua` to load the new filetypes module

.config/nvim/lua/core/filetypes.lua

@@ -0,0 +1,5 @@
+vim.filetype.add({
+   extension = {
+      njk = 'jinja',
+   },
+})

.config/nvim/lua/core/init.lua

@@ -1,3 +1,4 @@
-require "core.options"
+require('core.options')
-require "core.keymap"
+require('core.keymap')
-require "core.autocmds"
+require('core.autocmds')
+require('core.filetypes')