Add IndexOf function to ItemsSourceView.

[marcelwgn]

Description

This PR adds the IndexOf function.

API spec (in progress): microsoft/microsoft-ui-xaml-specs#74

Motivation and Context

Closes #1828

How Has This Been Tested?

Not tested so far, how should this be tested?

Screenshots (if appropriate):

[StephenLPeters]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[StephenLPeters]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[marcelwgn]

During API review @oldnewthing asked about passing null to IndexOf. We came to the conclusion that passing null should be valid. However the current implementation simply returns -1 if we pass null, regardless whether null is in the collection or not. Should I update this PR to also handle null as outlined in the API review or was there a specific reason to treat null different here?

[ranjeshj]

During API review @oldnewthing asked about passing null to IndexOf. We came to the conclusion that passing null should be valid. However the current implementation simply returns -1 if we pass null, regardless whether null is in the collection or not. Should I update this PR to also handle null as outlined in the API review or was there a specific reason to treat null different here?

Let's wait until the api review PR is closed before making this update in case we decide otherwise.

[ranjeshj]

During API review @oldnewthing asked about passing null to IndexOf. We came to the conclusion that passing null should be valid. However the current implementation simply returns -1 if we pass null, regardless whether null is in the collection or not. Should I update this PR to also handle null as outlined in the API review or was there a specific reason to treat null different here?

Let's wait until the api review PR is closed before making this update in case we decide otherwise.

@chingucoding Let's handle the null case now that we have gone through the review. Also, please merge in master to avoid the timeout issue. Thanks!

[marcelwgn]

Thanks for reminding, add test and support for null values now.

[ranjeshj]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[StephenLPeters]

[kmahone]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).