Add Buttons to Rails ActionText toolbar

What is the best way to configure ActionText to add buttons to the toolbar. For example, an h2 and h3 button?

To add to Will's answer, you can avoid recreating the whole toolbar by using trix after initialize callback :

example, adding a 'Red' button that turns the text red :

var Trix = require("trix");

/* what the newly created button does */
Trix.config.textAttributes.red = {
    style: { color: "red" },
    parser: function(element) {
        return element.style.color === "red"
    },
    inheritable: true
}

/* insert the button visual in the default toolbar */
addEventListener("trix-initialize", function(event) {
    var buttonHTML = '<button type="button" data-trix-attribute="red">RED</button>'

    event.target.toolbarElement.
    querySelector(".trix-button-group").
    insertAdjacentHTML("beforeend", buttonHTML)
})

output: