fix(@angular-devkit/build-angular): fix index option const value for browser-esbuild

[leosvelperez]

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:

What is the current behavior?

The index option of the browser-esbuild builder is defined to accept one of a string, an object, or a const "false". Note the const value it's not false but a string value. Because of that, there's no way to disable the index generation:

• If we set the option to "false" we get an error because it matches both the string and the const:

   Error: Schema validation failed with the following errors:
   Data path "/index" must be object.
   Data path "/index" must match exactly one schema in oneOf.
  

• If we set the option to false we get an error because it doesn't match any of the possible definitions:

   Error: Schema validation failed with the following errors:
   Data path "/index" must be string.
   Data path "/index" must be object.
   Data path "/index" must be equal to constant.
   Data path "/index" must match exactly one schema in oneOf.
  

• If we run the build with ng build --no-index (as one of the goals of the change 0d4a40f) we get the same as the previous error:

  Error: Schema validation failed with the following errors:
  Data path "/index" must be string.
  Data path "/index" must be object.
  Data path "/index" must be equal to constant.
  Data path "/index" must match exactly one schema in oneOf.
  

Issue Number: N/A

What is the new behavior?

The index option of the browser-esbuild builder is defined to accept one of a string, and object or a const false (boolean). This allows users to set the option to false in their angular.json or to run the build with --no-index.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[alan-agius4]

LGTM

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