feat: add secondary entrypoint schematic

[Jefiozie]

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• The commit message follows our guidelines: https://github.com/angular/angular-cli/blob/main/CONTRIBUTING.md#-commit-message-guidelines
• 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
• 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
• 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

[SanderElias]

Looks good to me! Ship It!

[alan-agius4]

Thank you for this. We discussed this during yesterday's meeting and agreed to proceed with adding a schematic to generate additional library entry points.

As a side note, it would be great if the library schematic could also leverage this schematic to generate its own entry point.

I’ve completed an initial review and left a few minor comments. Let me know if you have any questions!

[alan-agius4]

This might confuse users as this file should not be used to put code.

We we instead generate a service, component and module. IE: move the code from the library schematic

angular-cli/packages/schematics/angular/library/index.ts

Lines 168 to 193 in 518ff9f

	options.standalone
	? noop()
	: schematic('module', {
	name: options.name,
	commonModule: false,
	flat: true,
	path: sourceDir,
	project: packageName,
	}),
	schematic('component', {
	name: options.name,
	selector: `${prefix}-${options.name}`,
	inlineStyle: true,
	inlineTemplate: true,
	flat: true,
	path: sourceDir,
	export: true,
	standalone: options.standalone,
	project: packageName,
	}),
	schematic('service', {
	name: options.name,
	flat: true,
	path: sourceDir,
	project: packageName,
	}),

into this. This way the library schematic can use this schematic to generate its entry-point.