Indexer, collection, event and sublevel initialization

I propose we allow indexers, collections, and events to be assigned during construction. Currently, only properties and fields are allowed. An indexer is a property too, why can't we assign to it during construction?

As an extension to https://github.com/fsharp/fslang-suggestions/issues/877, I propose we allow:
```fs
let p = Builder([2] = initializeToIndexer, collection @= [addToCollection1; addToCollection2], event @= [addToEvent1; addToEvent2], Property = (SubLevelProp1 = 2, SubLevelProp2 = 1))
```

The existing way of approaching this problem in F# is
```fs
let p = Builder()
p.[2] <- initializeToIndexer
p.collection.Add addToCollection1
p.collection.Add addToCollection2
p.event.Add addToEvent1
p.event.Add addToEvent2
p.Property.SubLevelProp1 <- 2
p.Property.SubLevelProp2 <- 1
```
## Pros and Cons

The advantages of making this adjustment to F# are
1. Conciseness
2. Convenience - property initialization works with more things!

The disadvantages of making this adjustment to F# are that `@=` may have already been defined in user code. However, as with `nameof`, this can be solved by a library intrinsic.

## Extra information

Estimated cost (XS, S, M, L, XL, XXL): M

Related suggestions: https://github.com/fsharp/fslang-suggestions/issues/877

## Affidavit (please submit!)

Please tick this by placing a cross in the box:
* [x] This is not a question (e.g. like one you might ask on [stackoverflow](http://stackoverflow.com)) and I have searched stackoverflow for discussions of this issue
* [x] I have [searched both open and closed suggestions on this site](http://github.com/fsharp/fslang-suggestions/issues) and believe this is not a duplicate
* [x] This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it.

Please tick all that apply:
* [x] This is not a breaking change to the F# language design
* [x] I or my company would be willing to help implement and/or test this

## For Readers

If you would like to see this issue implemented, please click the :+1: emoji on this issue. These counts are used to generally order the suggestions by engagement.

Start by reviewing the proposed construction syntax and the related suggestion in issue 877, then inspect the F# language-design implementation and test areas identified by maintainers. The issue names no files, tests, or entry points; done would require an agreed design and corresponding implementation and tests for indexer, collection, event, and sublevel initialization.

Written by the indexing model from the issue text.