Add syntax highlighting for nullsafe member calls (#90)

Adds missing syntax highlighting for nullsafe method calls (`?->`).

Complements the existing `nullsafe_member_access_expression` rule with
`nullsafe_member_call_expression` for complete PHP 8.0 nullsafe operator
support.

Author: syntlyx

languages/php/highlights.scm

@@ -25,6 +25,9 @@
 (member_call_expression
   name: (name) @function.method)
 
+(nullsafe_member_call_expression
+  name: (name) @function.method)
+
 (function_definition
   name: (name) @function)