Add higher-order-function-based API for working with untyped AST (#16462)

* Add module-based API for working with untyped AST

* Fantomas

* tryPickUntil → tryPickDownTo

* Don't need that

* Thread path while walking

* Update comment

* Simplify

* Expose `Ast.fold` and `Ast.tryPick`.
* Expose `SyntaxNode.(|Attributes|)`.
* Ensure a few more syntax node cases get hit.

* Update FCS release notes

* Update surface area

* Add back `foldWhile`; add `exists`, `tryNode`

* Put `Ast.foldWhile` back in.

* Add `Ast.exists`.

* Add `Ast.tryNode`.

* `SyntaxTraversal.Traverse` → `Ast.tryPick`…

* Replace uses of `SyntaxTraversal.Traverse` in `FSharpParseFileResults`
  with the appropriate function from the `Ast` module: `exists`,
  `tryPick`, `tryNode`.

* Update surface area

* Need that

* Just to be safe

* Add `Ast.tryPickLast`

* Handle multiple args mid-pipeline

* Before, no signature help was offered in a case like this:

  ```fsharp
  [1..10]
  |> List.fold (fun acc _ -> acc) ‸
  |> List.filter (fun x -> x > 3)
  ```

  The service will now offer help for the `state` parameter when the
  cursor ‸ is in that location.

* `*` instead of error

* `FSharpParseFileResults.TryRangeOfFunctionOrMethodBeingApplied` was
  previously returning the range of the (zero-width)
  `SynExpr.ArbitraryAfterError`. It now returns the range of the `*`
  (`op_Multiply`) instead.

* Update surface area

* Fmt

* Missed in merge

* Add VS release notes entry

* # → ###

* Add 	ryPick tests

* Add a few more tests

* \n

* Bump release notes

* Fmt

* `Ast` → `ParsedInput`

* Use `ParsedInput` as the main AST type.

* Move the `position` parameter rightward.

* Update surface area

* Less `function`

* Update untyped AST docs

* Add basic examples for `ParsedInput` module functions.

* Merge the existing `SyntaxVisitorBase` docs into the new file.

* Clean up doc comments

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Co-authored-by: Vlad Zarytovskii <[email protected]>

Author: brianrourkeboll