[Bug]: delegate default entry assigning to core

[anshumanv]

Describe the bug

Presently we assign our own default entry which is invalid since it can be ts/mjs etc so delegate this to core, we only need to assign entry when user passes

What is the current behavior?

Entry assigned to index.js or src/index.js

To Reproduce
Add some non js index file then it won't work

Expected behavior
It should work

Screenshots

Please paste the results of webpack-cli info here, and mention other relevant information

Additional context

[alexander-akait]

@anshumanv Can you send a PR, it is very important

[anshumanv]

yep on it 👍