fix: keep the site cache purge token out of the page payload (NUXT_CACHE_PURGE_TOKEN)

nuxt-site-config copies every NUXT_SITE_* variable into the public site
config, so NUXT_SITE_CACHE_PURGE_TOKEN reached every page's payload as
cachePurgeToken (and NUXT_SITE_CACHE_ALLOW_PROXIED as cacheAllowProxied).

- The purge settings move to runtimeConfig cachePurge.token
  (NUXT_CACHE_PURGE_TOKEN) and cachePurge.allowProxied
  (NUXT_CACHE_PURGE_ALLOW_PROXIED), outside that prefix.
- The old siteCache.* names keep working for one release: a non-empty
  new token wins, either allowProxied flag turns it on, and one
  deprecation warning per process names the variables (never a value).
- The shared Nitro plugin server/plugins/site-cache-purge.ts strips
  cachePurgeToken and cacheAllowProxied from nuxt-site-config's stack on
  site-config:init, so a site on the old names leaks nothing either; it
  does nothing without nuxt-site-config.
- The shared lib, route and plugin and tests/sitePurge.test.ts are
  byte-identical in vue-journal-layer, vue-menu-layer and vue-faq-layer,
  with three sha256 pins.
- README: new names, the deprecation, the stripping and the advice to
  rotate a token ever set under the old name.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "vue-menu-layer",
-  "version": "0.3.0",
+  "version": "0.4.0",
   "private": true,
   "type": "module",
   "main": "./nuxt.config.ts",