feat: add secondary entrypoint schematic

[Jefiozie]

PR Checklist

Please check to confirm your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/angular/angular-cli/blob/main/CONTRIBUTING.md#-commit-message-guidelines
• [x] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• [ ] Bugfix
• [x] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [ ] Documentation content changes
• [ ] Other... Please describe:

What is the current behavior?

There is no easy way to generate a secondary entrypoint

Issue Number: #19070

What is the new behavior?

This PR adds the ability to generate a secondary entrypoint in a library project. It cannot be used without a library project.

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information

I've copied some helper utils from other schematics. I'm happy to make a other PR to add these to the utility parts with proper tests on it. Not updated the docs as this is I think in a other repo?

Pinging @sanderelias he mentioned there was nothing in the cli and encourage me to make this change 👍

Closes #19070

[Jefiozie]

Hi @alan-agius4 I saw that you added the discussion label. Any pointers what the discussion is about? I'm happy to join the discussion to see what we can improve or my point of view on things.

[alan-agius4]

@Jefiozie, since this is adding a new public API, we need to discuss this internally, and make sure that the team is on board with this.

[Jefiozie]

@alan-agius4 that does make sense, I will wait then and rebase this PR until you / the team comes back with feedback.

[Jefiozie]

@alan-agius4 the PR is up-to-date with latest changes

[Jefiozie]

Hi @alan-agius4, happy new year! Did you found the time to talk about this schematic with the team?

[alan-agius4]

Hi @Jefiozie,
Happy New Year to you as well! Apologies for the delay – with the v19 release and the holiday season, we haven't had a chance to discuss this PR yet. I’m hoping to do it this week.

[alan-agius4]

@Jefiozie, do you need help to make this green?

[Jefiozie]

Hi @alan-agius4 I've not yet had the time to finish this but I will have a look in the upcoming week. Thanks for the ping

[alan-agius4]

Closing as this PR is stalled. Feel free to re-open when you have time to continue on this.

[Jefiozie]

Thanks @alan-agius4 I just could not find enough time to work on it

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