✨ Code Completion Provider

# Overview
A completionProvider interface is needed to allow extensions to add code completion suggestions.

## Related API Documentation
- [CompletionProvider]( https://github.com/CodeEditApp/documentation/blob/main/api-reference/completion-provider.md)

## Resources
- https://code.visualstudio.com/api/references/vscode-api#CompletionItemProvider%3CT%3E
- https://github.com/microsoft/vscode-extension-samples/tree/main/completions-sample

Start with the linked CompletionProvider API documentation, then compare the VS Code CompletionItemProvider reference and completions sample. Define what extensions must provide and how completion suggestions should be exposed; done means an extension can add code completion suggestions through the new interface.

Written by the indexing model from the issue text.