Update the checklist for new language features

Change-Id: I4022bd3a79be25d006178ad681effd035ff7f9d0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/277045
Reviewed-by: Samuel Rawlins <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: bwilkerson

pkg/analysis_server/doc/process/new_language_feature.md

@@ -42,5 +42,6 @@ The features are listed in alphabetical order.
 - [ ] selectionRange
 - [ ] Semantic Highlights
 - [ ] signatureHelp
+- [ ] Snippets
 - [ ] Sort Members
 - [ ] Type Hierarchy