refactor(tutor): use inline comments instead of json file #39714

Problem: Expected text for interactive marks is in a separate json file
from the tutor file. When the tutor file is updated, line numbers
(potentially many) have to be updated in the json file. This is a
burden for maintenance and automatic testing.

Solution: Put the expected text inline in the tutor file, marked
with `[[]]`. Parse and remove the comments before opening the tutor
file so extmarks can be applied.

runtime/autoload/tutor.vim

@@ -12,10 +12,9 @@ function! tutor#SetupVim()
     endif
 endfunction
 
-" Loads metadata file, if available
+" Load inline metadata
 function! tutor#LoadMetadata()
-    let l:metadata_file = exists('b:tutor_file') ?  b:tutor_file : expand('%')
-    let b:tutor_metadata = json_decode(readfile(l:metadata_file .. '.json'))
+    lua require('nvim.tutor').load_metadata()
 endfunction
 
 " Mappings: {{{1