Add schema validation for plugin configSections

[garrytrinder]

If we introduce schema to the configSection, we should validate that the configSection is valid and include completions to help developers updated the schema with valid properties.

We can use vscode.languages.registerCompletionItemProvider and new vscode.CompletionItem() to add to the list of completions when the menu is opened.

We can use ajv to validate the JSON in the configSection against a schema.