Support base methods in closures

I propose we support
```fs
open System
type B() =
abstract K : int -> unit
default _.K _ = ()
type C() =
inherit B()
override _.K _ = ResizeArray().ForEach(Action<_> base.K)
// error FS0408: The 'base' keyword is used in an invalid way. Base calls cannot be used in closures. Consider using a private member to make base calls.
// error FS0419: 'base' values may only be used to make direct calls to the base implementations of overridden members
```

The existing way of approaching this problem in F# is making a private member as suggested.

However, the C# compiler does this for us and we don't have to workaround it.
https://sharplab.io/#v2:CYLg1APgAgzABFATHAQnA3g+A3AlgJwBcBXAQwBsEAWOAaQApcA7QuAfQEoMBfObgWABQsBMgDCcEKgxC4crHAD22AKb58uYCup1GLdl3Sz5JpioDuCAIxUAdABlcAZ0IAeZoQB89DrYBiivgAoqQAxgAW9ABGpE4qtrQcANzGcgKC3EA===

F# can do the same.

## Pros and Cons

The advantages of making this adjustment to F# are
1. Conciseness
2. Convenience
3. Consistency with C#

The disadvantage of making this adjustment to F# is that this may be an already-made decision in previous versions of F#.

## Extra information

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

Related suggestions: (put links to related suggestions here)

## 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
* [ ] 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 reproducing the F# example and comparing its closure behavior with the linked SharpLab C# example. Define the language-design and compatibility implications, then verify that base method calls work inside closures without the current FS0408 and FS0419 errors.

Written by the indexing model from the issue text.