Support returning a truncated set of completions using isIncomplete=true

[DanTup]

Outstanding issues:

• VS Code shows stale results (possibly due to cancellation of intermediate requests) during typing CompletionList.isIncomplete=true results in showing "stale" results when typing quickly due to cancelled requests microsoft/vscode#147830
• VS Code doesn't filter items client-side during typing Filter completion client-side even if the LSP returns incomplete results microsoft/vscode#168329
• Server truncates based on rank but doesn't guarantee exact matches are always included
• Server ranking tables are stale and may not rank the items you'd expect highly Bad initial completion ranking (no prefix) for named arguments #4618

[DanTup]

Added in dart-lang/sdk@bdf8150. Can be controlled by the dart.maxCompletionItems setting. Currently defaults to 2000, but this value is likely to change based on feedback and perf work.

[DanTup]

This is effectively turned off by 82b9ec7 and will need re-enabling when issues are resolved.