refactor(@angular-devkit/build-angular): convert transformedContent into a function

[d3lm]

This PR converts transformedContent into a function to give more control when the readable stream is initialized. There might be issues with late "subscribers" where events are added after the readable stream was initialized and the data was already pushed, e.g.

const { rewriter } = await htmlRewritingStream('<div />');

setTimeout(() => {
  rewriter.on('startTag', (tag) => {
    console.log(tag);
  });
})

This will not work and potentially wait indefinitely. I have ran into such issue with WebContainer on stackblitz.com.

With this change the stream now needs to be explicitily initialized:

const { rewriter, transformedContent } = await htmlRewritingStream('<div />');

setTimeout(() => {
  rewriter.on('startTag', (tag) => {
    console.log(tag);
  });
  
  // calling this function will create the readable stream
  transformedContent();
})

This will work as expected.

PR Checklist

Please check to confirm your PR fulfills the following requirements:

• 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:

Does this PR introduce a breaking change?

• Yes
• No

[alan-agius4]

LGTM thanks @d3lm. Always great working with you :)

[d3lm]

Thanks @alan-agius4 Likewise! Always good to be able to contribute to Angular / Angular CLI.

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