Provide types for `onPreferencesChanged` string

[benvillalobos]

In the handler for onPreferencesChanged, it would be great to be able to parse the string using known types provided by the SDK. Today, the VSCode for Education Extension uses manually created types based on the preferences string we received at the time. If they change at any point, the current method of fixing it would be to once again manually take a look at the string rather than updating a package to obtain the new types.

At minimum, the documentation, could define what's expected in the onPreferencesChanged string.

Some of the types returned in the prefs string were documented, but not all. Below is an example.

export type TranslationLanguageData = {
    tlc: string;
    slc: string;
};

export type TranslationLanguage = {
    key: string;
    text: string;
    data: TranslationLanguageData;
};

See https://github.com/microsoft/vscode-edu-extension/pull/1532/files for context. If you need access to the repository, please comment or message offline.