Changed way the default syntax highlighting lexer is overridden for PHP

Instead of using hardcoded list of language codes that trigger the PHP lexer, an `isinstance` is now used.

If Pygments decide to use PhpLexer, we detect it using isinstance and use our own PhpLexer instead.

tildes/tildes/lib/markdown.py

@@ -206,13 +206,12 @@ def apply_syntax_highlighting(html: str) -> str:
         language = code_block["class"][0].replace("language-", "")
 
         try:
+            lexer = get_lexer_by_name(language)
             # If target language is PHP, override default lexer construction
             # and set startinline to True, so even code that is not enclosed
             # inside <?php ... ?> will get highlighted.
-            if language in ["php", "php3", "php4", "php5"]:
+            if isinstance(lexer, PhpLexer):
                 lexer = PhpLexer(startinline=True)
-            else:
-                lexer = get_lexer_by_name(language)
         except ClassNotFound:
             continue