Template-level `@preserveWhitespace` "annotation"

[matanlurey]

For example:

<div>
  Collapse whitespace
  <div @preserveWhitespace>Preserve  All  Whitespace</div>
</div>

[matanlurey]

Spent ~15 minutes on this, mostly gathering requirements:

• Would be nice to do Refactor out a HasChildrenAst #1460 first
• EmbeddedTemplateAst needs support for .annotations
• MinimizeWhitespaceVisitor uses RecursiveAstVisitor, so you need to bail out (no super.)

Some example test cases:

test('should skip nodes annotated with @preserveWhitespace', () {
  expect(
    _parseAndMinifiy(
      r'<div @preserveWhitespace>   <span>  </span></div>',
    ),
    '<div @preserveWhitespace>   <span>  </span></div>',
  );
  expect(
    _parseAndMinifiy(
      r'<ng-container @preserveWhitespace>   </ng-container>',
    ),
    '<ng-container @preserveWhitespace>   </ng-container>',
  );
  expect(
    _parseAndMinifiy(
      r'<template @preserveWhitespace><div>  </div></template>',
    ),
    '<template @preserveWhitespace><div>  </div></template>',
  );
});

[matanlurey]

Fixed in #1464.