[client-js] Update TypeScript build target to ES2020

[santitigaga]

Description

This pull request updates the library’s TypeScript configuration to target ES2020.
The change modernizes the compiled output, enabling the use of newer JavaScript features while keeping compatibility with current Node.js and bundler ecosystems.

Key changes

Updated target in tsconfig.json from es2015 to es2020.
Ensures the generated artifacts use more modern syntax and improved performance capabilities.
No breaking changes to the API; only build output is affected.

Motivation

Modernizing the output to ES2020 allows:

Better optimization by bundlers.
Reduced transpilation overhead.
Usage of more efficient native JavaScript features available in ES2020 environments.

Category

• Feature
• Fix

Checklist

• The pull request is complete according to the guide of contributing
• Automated tests are written
• The documentation is up-to-date
• the version of the mix.exs was increased
• The pull request has a descriptive title that describes what has changed, and provides enough context for the changelog

[sonarqubecloud]

Quality Gate passed

Issues
22 New issues
0 Accepted issues

Measures
1 Security Hotspot
81.7% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[juan-campuzano]

Thanks!