Fix memory leak (#1718)

Author: ota-meshi

lib/utils/style-variables/index.js

@@ -32,8 +32,8 @@ module.exports = {
   StyleVariablesContext
 }
 
-/** @type {Map<VElement, StyleVariablesContext>} */
-const cache = new Map()
+/** @type {WeakMap<VElement, StyleVariablesContext>} */
+const cache = new WeakMap()
 /**
  * Get the style vars context
  * @param {RuleContext} context