Avoid adding duplicate items to 'treesit-language-source-alist'.

Avoid adding duplicate items to 'treesit-language-source-alist'.

* lisp/textmodes/mhtml-ts-mode.el:
* lisp/progmodes/php-ts-mode.el:
Use 'add-to-list' on items in language-source-alist
that should check for duplicate items.

* test/infra/Dockerfile.emba: Move multi-language mhtml-ts-mode and
php-ts-mode to the end to give their dependent grammars a lower priority.

* test/lisp/progmodes/js-tests.el (js-ts-mode-test-indentation):
Skip the test when 'jsdoc' is missing since 'js-ts-mode' depends
on the 'jsdoc' grammar.

(cherry picked from commit 5684a3808677fb701833c83325cd3bc8de475561)