vim-patch:9.2.0500: filetype: some html files wrongly recognized as htmlangular (#39880)

Problem:  filetype: some html files are wrongly recognized as htmlangular
Solution: Use the \< atom to anchor ng-template and ng-content to start
          of word (truffle)

Prevent false-positive htmlangular detection on words containing
'ng-template' or 'ng-content' as a substring (e.g. 'song-template',
'sing-content'). Anchor both branches with \< to require a word start,
matching the \<DTD\s\+XHTML\s idiom used five lines below.

related: neovim/neovim#39778.
closes:  vim/vim#20246

https://github.com/vim/vim/commit/354ab1a69efbe5f84cd7225bc3945db3f2cec5c4

Co-authored-by: truffle <truffleagent@gmail.com>
(cherry picked from commit f3bb21e71d)

runtime/lua/vim/filetype/detect.lua

@@ -842,7 +842,7 @@ function M.html(_, bufnr)
     if
       matchregex(
         line,
-        [[@\(if\|for\|defer\|switch\)\|\*\(ngIf\|ngFor\|ngSwitch\|ngTemplateOutlet\)\|ng-template\|ng-content]]
+        [[@\(if\|for\|defer\|switch\)\|\*\(ngIf\|ngFor\|ngSwitch\|ngTemplateOutlet\)\|\<ng-template\|\<ng-content]]
       )
     then
       return 'htmlangular'

test/old/testdir/test_filetype.vim

@@ -1869,6 +1869,27 @@ func Test_html_file()
   call assert_equal('htmlangular', &filetype)
   bwipe!
 
+  " HTML Angular ng-template element
+  let content = ['<ng-template let-foo>{{ foo }}</ng-template>']
+  call writefile(content, 'Xfile.html', 'D')
+  split Xfile.html
+  call assert_equal('htmlangular', &filetype)
+  bwipe!
+
+  " HTML Angular ng-content element
+  let content = ['<div><ng-content select="[item]"></ng-content></div>']
+  call writefile(content, 'Xfile.html', 'D')
+  split Xfile.html
+  call assert_equal('htmlangular', &filetype)
+  bwipe!
+
+  " Word containing 'ng-template' as a suffix must not trigger htmlangular
+  let content = ['<div class="song-template">', '  <h1>Not Angular</h1>', '</div>']
+  call writefile(content, 'Xfile.html', 'D')
+  split Xfile.html
+  call assert_equal('html', &filetype)
+  bwipe!
+
   " Django Template
   let content = ['{% if foobar %}',
       \ '    <ul>',