feat(content): add agx support for injectContent

[joshuamorony]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/analogjs/analog/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:

Which package are you modifying?

• vite-plugin-angular
• vite-plugin-nitro
• astro-angular
• create-analog
• router
• platform
• content
• nx-plugin
• trpc

What is the current behavior?

Using a .agx file currently requires manually importing/creating the component, e.g:

  defineMetadata({
    queries: {
      divPost: new ViewChild('post', { static: true, read: ViewContainerRef }),
    }
  });

  let divPost: ElementRef<HTMLDivElement>;
  const cdr = inject(ChangeDetectorRef);

  afterNextRender(() => {
    const postId = 'post';
    import(`../../content/${postId}.agx`).then(m => {
      divPost.createComponent(m.default);
      cdr.detectChanges();
    });
  });

What is the new behavior?

.agx files can be used in the same manner as .md files with injectContent, e.g:

  const post$ = injectContent({
    customFilename: 'post'
  });
  const post = toSignal(post$)

<analog-markdown [content]="post().content"></analog-markdown>

Does this PR introduce a breaking change?

• Yes
• No

Other information

[optional] What gif best describes this PR or how it makes you feel?

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[nartc]

Some comments