refact(prompt_title): move strip_extension to utils

lua/telekasten/utils.lua

@@ -1,18 +1,27 @@
 local M = {}
 
-function M.prompt_title(callback)
+-- strip an extension from a file name, escaping "." properly, eg:
+-- strip_extension("path/Filename.md", ".md") -> "path/Filename"
+local function strip_extension(str, ext)
+    return str:gsub("(" .. ext:gsub("%.", "%%.") .. ")$", "")
+end
+
+function M.prompt_title(ext, callback)
     local canceledStr = "__INPUT_CANCELLED__"
 
     vim.ui.input({
         prompt = "Title: ",
-        default = "",
         cancelreturn = canceledStr,
     }, function(title)
+        if not title then
+            title = ""
+        end
         if title == canceledStr then
             vim.cmd("echohl WarningMsg")
             vim.cmd("echomsg 'Note creation cancelled!'")
             vim.cmd("echohl None")
         else
+            title = strip_extension(title, ext)
             callback(title)
         end
     end)