Allow .tsx and .jsx files as entry modules.

[shirakaba]

Accommodates React NativeScript (and any other project that might wish to use TSX/JSX syntax for whatever reason).

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-dev-webpack/blob/master/CONTRIBUTING.md#testing-locally-by-running-e2e-tests
• Tests for the changes are included.

What is the current behavior?

What is the new behavior?

Fixes/Implements/Closes #[Issue Number].

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[rosen-vladimirov]

Hey @shirakaba ,
I'm closing this draft PR as there's no work on it for a long time.