Support `defaultSnippets` VSCode's custom keyword

[Ahmedmhmud]

Related to #85

VS Code supports the custom keyword defaultSnippets. We want to support it too, so users can move to this language server more easily.

Scope

• Add a new type in CompletionEvaluationPlugin.ts to store defaultSnippets data in a simple way.
• Add a separate function in CompletionEvaluationPlugin.ts to collect defaultSnippets from schema nodes during validation.
• Add a new async wrapper in JsonDocument.ts to get snippet data using the current JSON pointer, the same way the existing completion helpers work.
• Use that data in PropertyCompletion.ts to build snippet completion items.
• Add tests for the new behavoir.

[Ahmedmhmud]

After studying the normal completion flow, I think this is the best way to implement this keyword while staying consistent with the repo. If that’s okay, I’d like to work on it.

[jdesrosiers]

I don't want to add anything to CompletionEvaluationPlugin if we don't have to. It's already way too complex.

The new keyword is just an annotation, so we should be able to retrieve it using the AnnotationsEvaluationPlugin based on its pointer and the keyword URI. However, we don't collect annotations for values that don't exist yet, which means they won't be available for the typical completion scenario. I think we can do something similar to what we did for CompletionEvaluationPlugin where we pass in the incomplete locations. Then the AnnotationsEvaluationPlugin can be updated to collect annotations for those locations as well. Then we can just look up any annotation keywords we need when building CompletionItems.

Use that data in PropertyCompletion.ts to build snippet completion items.

Not property completions. This would apply to value completions.

See the discussion in #87 about keyword URIs and vocabulary registration. The same will apply for this keyword.

[Ahmedmhmud]

Alright I read the discussion and ready to go with the new approach which is the best fit for this one.