@W-15207087@ Update entry point of the project

[mingxuanzhangsfdx]

What does this PR do?

1. Update the entry point of the project from exported.ts to index.ts
2. Rename the main field and types declaration for the entry point name change
3. Update the bundling script so that

• the generated file name aligns with package.json
• the hardcoded package name for Messages.loadMessages is '@salesforce/core-bundle'

What issues does this PR fix or reference?

@W-15207087@

https://gus.lightning.force.com/lightning/r/ADM_Work__c/a07EE00001lh6OPYAY/view

Due to the limitation of npm-dts the entry file should be index.ts otherwise it will refer to a wrong path of the module in the generated type declarations.

[shetzel]

Looks fine to me.