Add import path high-lighting

I don't think this group is supported by neovim, but 'tree-sitter-nix'
makes use of it, and I think it makes sense to add it.

queries/highlights.scm

@@ -65,6 +65,11 @@
   name: (identifier) @variable.parameter)
 ; }}}
 
+; Declarations {{{
+(import_declaration
+  file: (string_literal) @string.special.path)
+; }}}
+
 ; Literals {{{
 (nil_literal) @constant.builtin
 (integer_literal) @number

test/highlight/imports.tig

@@ -0,0 +1,3 @@
+import "lib.tih"
+/* <- keyword */
+/*      ^ string.special.path */