Add JSON program generation, validation, and evaluation in core module

[ahejlsberg]

This PR adds support for generation, validation, and evaluation of simple programs targeting a specified API. Programs consist of a sequence of calls to functions of the specified API with arguments that are a combination of JSON values and results from preceding program steps. Programs are are represented as JSON using the following schema:

// A program consists of a sequence of expressions that are evaluated in order.
export type Program = {
    "@steps": Expression[];
}

// An expression is a JSON value, a function call, or a reference to the result of a preceding expression.
export type Expression = JsonValue | FunctionCall | ResultReference;

// A JSON value is a string, a number, a boolean, null, an object, or an array. Function calls and result
// references can be nested in objects and arrays.
export type JsonValue = string | number | boolean | null | { [x: string]: Expression } | Expression[];

// A function call specifices a function name and a list of argument expressions. Arguments may contain
// nested function calls and result references.
export type FunctionCall = {
    // Name of the function
    "@func": string;
    // Arguments for the function
    "@args": Expression[];
};

// A result reference represents the value of an expression from a preceding step.
export type ResultReference = {
    // Index of the previous expression in the "@steps" array
    "@ref": number;
};

For example, given a music service API with functions getRecentlyPlayed, filterTracks, and createPlaylist, the user request "get my favorite 50 tracks from last year and make the non-classical ones into a playlist" might be represented as the following Program instance:

{
    "@steps": [
        {
            "@func": "getRecentlyPlayed",
            "@args": [{ "favoritesTerm": "long_term", "count": 50 }]
        },
        {
            "@func": "filterTracks",
            "@args": [{ "@ref": 0 }, { "filter": "genre:classical", "negate": true }]
        },
        {
            "@func": "createPlaylist",
            "@args": [{ "@ref": 1 }, "Non-Classical Favorites" ]
        }
    ]
}

The createProgramTranslator function introduced in this PR creates an object that can translate natural language requests into such programs. The translate method of the object combines the Program schema, a specified API schema, and a user request into a prompt that directs the LLM to generate a JSON program. The resulting program can be safely evaluated using the evaluateJsonProgram function, which implements a simple interpreter. Calls to API functions in the program are passed to an onCall callback function for validation and dispatch. Thus, unlike JavaScript's eval, access to external functionality and resources is entirely controlled by the host application.

[steveluc]

Great to see. Looks good. Shall we remove the programs example since it matches the music one at this point?

[ahejlsberg]

Yes, I'll put up a separate PR to remove examples/programs.