Tweak property and selector variables

As of ST4149 `&` is part of selector_start in CSS and doesn't need to be
explicitly added by Less.

Variable `selector_begin` is maintained only for backward compatibility
with some unreleased revisions of core CSS.

Variable `property_or_selector_begin` as adjusted to match CSS
(as of sublimehq/Packages#3898) and is maintained
for backward compatibility with ST builds which do not ship that PR.

Syntaxes/Less.sublime-syntax

@@ -16,13 +16,13 @@ variables:
 
   ident_start: (?:{{nmstart}}|@+{)
 
-  selector_begin: (?={{selector_start}}|&|@+{)
+  property_or_selector_begin: (?={{ident_begin}}|{{selector_start}})
+
+  selector_begin: (?={{selector_start}}|@+{)  # for backward compatibility only
   selector_end: (?=[;{}])
 
   tag_name_begin: (?=[[:alpha:]]|@+{)
 
-  property_or_selector_begin: (?={{selector_start}}|[-&]|{{ident_start}})
-
   # Less variables
 
   at_rules: |-