Add a workaround for SnippetTextEdit.keepWhitespace not being available in LSP #6120

We switched from non-standard snippet text edits to the new LSP v3.18 SnippetTextEdits, but it turns out LSP doesn't have a keepWhitespace field (which VS Code does), and so the whitespace is getting messed up.

I've opened microsoft/language-server-protocol#2290 to determine what the correct fix is, but in the meantime we should see if we can override this client-side in Dart-Code for now to avoid indentation for all code actions potentially being messed up.

The original bug report with steps to repro is at dart-lang/sdk#63790