fix(core): added @angular/compiler as a peer dependency

[emilrowland]

This fixes an issue with packages managers likes pmpm that will not link the @angular/compiler package to the @angular/core package if it is not listed as a peer dependency.
I added it as optional peer dependency as it's only used in special cases.

Fixes #38096

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/CONTRIBUTING.md#commit
• 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
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?

Today @angular/core/testing is using @angular/compiler without specifying it as a dependency.
This results in strict package managers like pnpm don't find the dependency.

Issue Number: #38096

What is the new behavior?

@angular/compiler is added as an optional peerDependency for @angular/core

Does this PR introduce a breaking change?

• Yes
• No

Other information

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[alan-agius4]

LGTM, one NIT.

For context, @angular/compiler is required by @angular/core/testing