fix: a couple of fixes and improvements on task --init (#2433)

* Fixed check for an existing Taskfile: look for all possibilities, and
  not only `Taskfile.yml` specifically.
* Added a description (`desc`) to the `default` task. Important to at
  least `task --list` work by default (a core feature).
* Changed top comment to YAML language server comment.

taskfile/templates/default.yml

@@ -1,12 +1,13 @@
-# https://taskfile.dev
+# yaml-language-server: $schema=https://taskfile.dev/schema.json
 
 version: '3'
 
 vars:
-  GREETING: Hello, World!
+  GREETING: Hello, world!
 
 tasks:
   default:
+    desc: Print a greeting message
     cmds:
       - echo "{{.GREETING}}"
     silent: true