Angular Select placeholder integration

[atmgrifter00]

Pull Request

🤨 Rationale

• Placeholder for nimble-select when nothing is selected #350

👩‍💻 Implementation

1. Expose the necessary Angular attributes for the NimbleListOptionDirection to support the placeholder use-case
2. Changes to nimble-components to address a problem discovered during early integration attempts for a particular SLE usage.
   • It was discovered that there are scenarios that will add nimble-list-options dynamically to the Angular template contents, followed immediately with setting a form value (that the Select was bound to) to the dynamically added option's value. This results in situation where the SelectControlValueAccessor processes the set value (which for the Select requires that option to be present on the web component side) prior to the Select adding the option to its set, which would result in the Select view ignoring that value and setting it to empty string instead.
   • The solution to this is to leverage the connectedCallback override of the NimbleListOption to allow it to have its parent, through an available interface if implemented, process it. All the Select does, is if the option isn't already part of its options, it simply pushes it on to it. There is no concern with this interfering with the normal process of the option being added as part of the slottedOptionsChanged callback, that will simply result in overwriting the options (which is fine).
   • Applied this solution to the combobox too
3. Exported the SelectPageObject from Angular, so that we can also leverage it in SLE as needed. Removed a lot of unnecessary async aspects of the SelectPageObject public methods.
4. Updated example app to use the feature and removed extra items from select to align with Blazor app and avoid Lighthouse performance degradation.

🧪 Testing

Added unit tests in nimble-components for the new internal interface the Select is using. Added the boilerplate unit tests in Angular for the new attributes. Also added a couple of Angular tests one of which is representative of the original failing scenario from the integration effort (and the other is more for completion).

✅ Checklist

• I have updated the project documentation to reflect my changes or determined no changes are needed.