Update Button params.type to match the documentation

Our documentation says Button components as `<button>` should always default `params.type` to `submit`:

> Type of `input` or `button` – `button`, `submit` or `reset`. Defaults to `submit`. This has no effect on `a` elements.

Rather than rely on `<button>` to implicitly default to `type="submit"` we should set it in the HTML

packages/govuk-frontend/src/govuk/components/button/template.njk

@@ -46,7 +46,7 @@ treat it as an interactive element - without this it will be
 </a>
 
 {%- elseif element == 'button' %}
-<button {%- if params.value %} value="{{ params.value }}"{% endif %}{%- if params.type %} type="{{ params.type }}"{% endif %} {{- buttonAttributes | safe }} {{- commonAttributes | safe }}>
+<button {%- if params.value %} value="{{ params.value }}"{% endif %} type="{{ params.type if params.type else 'submit' }}" {{- buttonAttributes | safe }} {{- commonAttributes | safe }}>
   {{ params.html | safe if params.html else params.text }}
 {# Indentation is intentional to output HTML nicely #}
 {{- iconHtml | safe | trim | indent(2, true) if iconHtml -}}