refactor: move filetype customization from polish.lua to astrocore.lua (#31)

Co-authored-by: Mark Weng <mweng@nvidia.com>

lua/plugins/astrocore.lua

@@ -24,6 +24,19 @@ return {
       virtual_text = true,
       underline = true,
     },
+    -- passed to `vim.filetype.add`
+    filetypes = {
+      -- see `:h vim.filetype.add` for usage
+      extension = {
+        foo = "fooscript",
+      },
+      filename = {
+        [".foorc"] = "fooscript",
+      },
+      pattern = {
+        [".*/etc/foo/.*"] = "fooscript",
+      },
+    },
     -- vim options can be configured here
     options = {
       opt = { -- vim.opt.<key>