Add assert keyword in completions

[gabritto]

Fixes #46371

I added a new function, getContextualKeywords, that is called in completionInfoFromData.
The alternative I've considered would be to use the existing keywordFilter mechanism, and have a new filter, KeywordFilter.Assert. However, that felt odd and difficult to implement, because keywordFilter is created in getCompletionData and assigned inside that function, and in other functions it calls, in multiple places. So it's (a) hard to follow all the conditionals and function calls that could assign to keywordFilter to make sure the variable ends up being assigned to KeywordFilter.Assert in the right places, and (b) the existing conditionals didn't reflect the cases where we should offer assert in completions.