Support CompletionList.isIncomplete=true on empty lists (or at least, clearly document current behaviour)

I was surprised to find that `CompletionList.isIncomplete=true` seems to be ignored when the result set is empty. The Dart server has the ability to place a maximum time on completions for not-currently-imported symbols to avoid slowing down all code completion to wait for these results (but typing further characters to narrow things down could include them). As far as I can tell, VS Code's current behaviour of ignoring `isIncomplete=true` when the list is empty could be simulated by returning `isIncomplete=false` when the list is empty (which makes sense, logically) so this limitation feels overly-restrictive. Additionally, the comment at https://github.com/microsoft/vscode/issues/13735#issuecomment-253753649 suggests that this is also the case if the _client-filtered_ list is empty. However, this is not what happens in my experience. If I insert a single item with label "FAKE" and then invoke completion with a prefix of "aaa", then `isIncomplete` _is_ still honoured, and further keystrokes result in additional trips to the server. So this is initially a request to support `isIncomplete=true` for empty lists, but if that's not going to change, I would at least like to see the current behaviour clearly documented (since LSP inherits this behaviour, and it's important LSP clients and servers have a good understanding of each others behaviour). I suspect there are other people that would like to see this: - @svenefftinge who originally asked about this in https://github.com/microsoft/vscode/issues/13735 - @ramya-rao-a asked about this at https://github.com/microsoft/vscode/issues/32423 but it's not clear what the resolution was - @ckant appears to have had to work around this in aws-toolkit at https://github.com/aws/aws-toolkit-vscode/commit/9f85fadab26fad65257559f594e448080262e975 (see the last part of the commit message) - @puh appears to have worked around this in https://github.com/puuuuh/crates/commit/81077a84e27921f92faef60020b6667a18baf223 in the same way by re…