chore: allow absolute prefix in parcel

.proxyrc.js

 const process = require('process')
 const serveStatic = require("serve-static")
 module.exports = function (app) {
+  console.error(('proxy config'))
   if (!('BUILD_PREFIX' in process.env)) {
     throw new Error(`Dunno where the chardata at, what BUILD_PREFIX?`)
   }
-  const dir = `./${process.env.BUILD_PREFIX}hanzi-data`
+  const dir = `${process.env.BUILD_PREFIX}hanzi-data`
   app.use("/hanzi-data", serveStatic(dir))
+  console.error(('ok configured at '), dir)
 }