go/ast: rename MultiIndexExpr to IndexListExpr

go/ast: rename MultiIndexExpr to IndexListExpr

As discussed in #47781, IndexListExpr is one character shorter and has
the advantage of being next to IndexExpr in documentation.

Updates #47781

Change-Id: I709d5c1a79b4f9aebcd6445e4ab0cd6dae45bab7
Reviewed-on: https://go-review.googlesource.com/c/go/+/348609
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>