Prior to this commit, an IndexAccessor could not provide support for a
Collection target type, since the built-in support for indexing into a
Collection in SpEL's Indexer took precedence.
This commit allows an IndexAccessor to support custom Collection target
types by separating the built-in List and Collection support and
applying the built-in Collection support after custom index accessors
have been applied.
Closes gh-32736
153d1bc923