FEAT: Plugin API - multi-field prompts and confirm dialogs

[andrinoff]

Is your feature request related to a problem?

matcha.prompt() only supports a single text input field. Plugins that need multi-step configuration (e.g. API key + model selection, or multiple form fields) must chain multiple prompts sequentially, which is awkward UX. There's also no way to ask a simple yes/no confirmation question.

Describe the solution you'd like

At minimum, add a confirm dialog:

matcha.confirm("Are you sure you want to proceed?", function(yes)
    if yes then
        -- do the thing
    end
end)

Ideally, also support multi-field prompts:

matcha.prompt_fields({
    {name = "api_key", placeholder = "Enter API key"},
    {name = "model", placeholder = "Model name (default: gpt-4)"},
}, function(fields)
    local key = fields.api_key
    local model = fields.model
end)

Describe alternatives you've considered

Chaining multiple matcha.prompt() calls in nested callbacks. This works but creates deeply nested callback hell and poor UX (multiple sequential popups).

Additional context

N/A