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

[DanTup]

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 #13735 (comment) 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 [completion] CompletionList#isIncomplete ignored on empty or fully filtered items #13735
• @ramya-rao-a asked about this at php: Emmet abbreviations with class operator dont show up in suggestions unless php.suggest.basic is disabled #32423 but it's not clear what the resolution was
• @ckant appears to have had to work around this in aws-toolkit at aws/aws-toolkit-vscode@9f85fad (see the last part of the commit message)
• @puh appears to have worked around this in puuuuh/crates@81077a8 in the same way by re-inserting a dummy entry containing the typed prefix

These are just the cases that mentioned the previous issue so they showed up on GitHub, I suspect there are many more.

The problem with the workaround of inserting the prefix is not just that it looks odd to users, but it doesn't work if there is no prefix. If the user hits Ctrl+Space and gets no items, the extension/server is never given any opportunity to provide completions as they type.