fix: tns update will not modify webpack.config.js of React/Svelte projects

[rosen-vladimirov]

As both React and Svelte projects have custom webpack.config.js files, when tns update replaces the webpack.config.js file, the project becomes unusable. We do not have default webpack.config.js files for these templates, so for the momennt skip the replacement of the file during tns update and print informational warning.

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/nativescript-cli/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
• Tests for the changes are included.

What is the current behavior?

Executing tns upate in React-nativescript or Svelte project replaces the webpack.config.js with default one for our JavaScript template. This makes the projects unusable and broken.

What is the new behavior?

CLI prints a warning during tns update that it will not update the webpack.config.js file as the project type is React or Svelte

Fixes comment #4947 (comment)