Fix volumeNetworks import

[tuyakhov]

When running scripts using ts-node, the library produces the following error:

ts-node scripts/curve.ts

Error [ERR_MODULE_NOT_FOUND]: Cannot find module '/MyProject/node_modules/@curvefi/api/lib/constants/volumeNetworks' imported from /MyProject/node_modules/@curvefi/api/lib/utils.js
    at new NodeError (node:internal/errors:387:5)
    at finalizeResolution (node:internal/modules/esm/resolve:330:11)
    at moduleResolve (node:internal/modules/esm/resolve:907:10)
    at defaultResolve (node:internal/modules/esm/resolve:1115:11)
    at nextResolve (node:internal/modules/esm/loader:163:28)
    at ESMLoader.resolve (node:internal/modules/esm/loader:841:30)
    at ESMLoader.getModuleJob (node:internal/modules/esm/loader:424:18)
    at ModuleWrap.<anonymous> (node:internal/modules/esm/module_job:76:40)
    at link (node:internal/modules/esm/module_job:75:36) {
  code: 'ERR_MODULE_NOT_FOUND'
}

A minimum reproducible example can be found here https://stackblitz.com/edit/stackblitz-starters-blfcep

This issue is caused by the missing .js extension for volumeNetworks import, which is also inconsistent with the rest of the imports in the repository.