Add LINQ APIs for Index and Range (#28776)

[Dixin]

Add LINQ APIs for index and range:

• Enumerable
  • ElementAt(Index)
  • ElementAtOrDefault(Index)
  • Take(Range)
• Queryable
  • ElementAt(Index)
  • ElementAtOrDefault(Index)
  • Take(Range)

Close#28776.

This is to replace PR #47950. I initially opened PR #47950, but accidentally closed it, and I cannot reopen it.

@stephentoub@eiriktsarpalis Please review this PR. Sorry for the inconvenience.

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

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Issue Details

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Add LINQ APIs for index and range:

• Enumerable
  • ElementAt(Index)
  • ElementAtOrDefault(Index)
  • Take(Range)
• Queryable
  • ElementAt(Index)
  • ElementAtOrDefault(Index)
  • Take(Range)

Close #28776.

This is to replace PR #47950. I initially opened PR #47950, but accidentally closed it, and I cannot reopen it.

@stephentoub@eiriktsarpalis Please review this PR. Sorry for the inconvenience.

Author:	Dixin
Assignees:	-
Labels:	area-System.Linq, new-api-needs-documentation
Milestone:	-

[eiriktsarpalis]

Thank you for your contribution!

[eiriktsarpalis]

CI failures seem unrelated, merging away.

[Dixin]

@eiriktsarpalis@stephentoub Thank you for all the help. As we discussed, I opened a new proposal for optimization #48631. I am already working on the coding, will send PR once it is done.