Highlight `unique` as a keyword

`unique` is really a context-sensitive keyword, but that's even more of a hassle in a TextMate grammars than it is in the compiler itself. We'll just highlight it as a real keyword. The worst that will happen is that existing variables and predicates named "unique" will be highlighted like keywords, which will hopefully just encourage QL developers to rename those anyway.

extensions/ql-vscode/syntaxes/ql.tmLanguage.yml

@@ -422,6 +422,11 @@ repository:
       keyword: 'true'
     name: constant.language.boolean.true.ql
 
+  unique:
+    match:
+      keyword: 'unique'
+    name: keyword.aggregate.unique.ql
+
   where:
     match:
       keyword: 'where'
@@ -478,6 +483,8 @@ repository:
     - include: '#then'
     - include: '#this'
     - include: '#true'
+    # `unique` is not really a keyword, but we'll highlight it as if it is.
+    - include: '#unique'
     - include: '#where'
 
   # A keyword that can be the first token of a predicate declaration.