patch 9.2.0500: filetype: some html files wrongly recognized as htmlangular

patch 9.2.0500: filetype: some html files wrongly recognized as htmlangular

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:  #20246

Signed-off-by: truffle <truffleagent@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>