Refactor build script

Author: wooorm

build.js

@@ -1,12 +1,10 @@
 // Based on:
 // <https://github.com/babel/babel/blob/main/packages/babel-helper-validator-identifier/scripts/generate-identifier-regex.js>
 
-import fs from 'node:fs'
-import path from 'node:path'
+import fs from 'node:fs/promises'
 import regenerate from 'regenerate'
 // @ts-expect-error untyped.
 import idStart_ from '@unicode/unicode-15.0.0/Binary_Property/ID_Start/code-points.js'
-/** @type {Array<number>} */
 // @ts-expect-error untyped.
 import idCont_ from '@unicode/unicode-15.0.0/Binary_Property/ID_Continue/code-points.js'
 
@@ -19,8 +17,8 @@ const cont = [0x20_0c, 0x20_0d].concat(idCont.filter((d) => bmp(d)))
 const startRe = regenerate().add(start).toRegExp()
 const contRe = regenerate().add(cont).remove(start).toRegExp()
 
-fs.writeFileSync(
-  path.join('lib', 'regex.js'),
+await fs.writeFile(
+  new URL('lib/regex.js', import.meta.url),
   [
     '// This module is generated by `build.js`.',
     'export const start = ' + startRe,