[CSS] Fix custom property definitions (#4052)

This commit fixes custom property definitions with declaration-list values.

Custom properties...
1. can take all sorts of values.
2. never start a selector.

Without this commit, leading `--` is ignored and custom property name being
consumed as html tag, starting a selector.

CSS/CSS.sublime-syntax

@@ -1550,6 +1550,7 @@ contexts:
 ###[ PROPERTY IDENTIFIERS ]####################################################
 
  properties-or-selectors:
+ - include: custom-properties
  - match: '{{property_or_selector_begin}}'
  branch_point: property-or-selector
  branch:

CSS/syntax_test_css.css

@@ -2167,6 +2167,21 @@
 }
 
 .test-var-functions {
+ --custom-theme: { color: white; };
+/*^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ meta.property-list.css meta.block.css */
+/* ^^^^^^^^^^^^^^ meta.property-name.css entity.other.custom-property.css */
+/* ^ punctuation.separator.key-value.css */
+/* ^ meta.property-value.css */
+/* ^^^^^^^^^^^^^^^^^ meta.property-list.css meta.block.css */
+/* ^ punctuation.section.block.begin.css */
+/* ^^^^^ meta.property-name.css support.type.property-name.css */
+/* ^ punctuation.separator.key-value.css */
+/* ^^^^^^ meta.property-value.css */
+/* ^^^^^ support.constant.color.w3c.standard.css */
+/* ^ punctuation.terminator.rule.css */
+/* ^ punctuation.section.block.end.css */
+/* ^ punctuation.terminator.rule.css */
+
  --test-var: arial;
 /* ^^^^^^^^^^ entity.other.custom-property.css */
 /* ^ punctuation.separator.key-value.css*/