Add async as dependency (required by sprite tool)

[ZakarFin]

Fixes an issue where running the sprite scripts fails with:

+ npm run sprite -- 1.28.1~dev-0.20241017_2137:applications

> [email protected] sprite
> node ./node_modules/oskari-frontend/webpack/sprite.js 1.28.1~dev-0.20241017_2137:applications

node:internal/modules/cjs/loader:1031
  throw err;
  ^

Error: Cannot find module 'async'
Require stack:
- /path/to/sample-application/oskari-frontend/webpack/sprite.js
    at Function.Module._resolveFilename (node:internal/modules/cjs/loader:1028:15)
    at Function.Module._load (node:internal/modules/cjs/loader:873:27)
    at Module.require (node:internal/modules/cjs/loader:1100:19)
    at require (node:internal/modules/cjs/helpers:119:18)
    at Object.<anonymous> (/path/to/sample-application/oskari-frontend/webpack/sprite.js:6:15)
    at Module._compile (node:internal/modules/cjs/loader:1198:14)
    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1252:10)
    at Module.load (node:internal/modules/cjs/loader:1076:32)
    at Function.Module._load (node:internal/modules/cjs/loader:911:12)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:81:12) {
  code: 'MODULE_NOT_FOUND',
  requireStack: [
    '/path/to/sample-application/oskari-frontend/webpack/sprite.js'
  ]