This commit introduces the HTML that will control the map’s zoom level.
The controls add <button> elements that contain inline-<svg>
plus and minus icons.
The elements are declared with type="button". While this
attribute feels redundant, it’s important to declare, since we expect to
extend them with JavaScript behavior.
According to the MDN Documentation, type="button" ensures that they
won’t submit <form> elements, should they find themselves
in a situation where <form> elements are their ancestors:
type="submit": The button submits the form data to the server. This
is the default if the attribute is not specified, or if the attribute
is dynamically changed to an empty or invalid value.
type="button": The button has no default behavior. It can have
client-side scripts associated with the element’s events, which are
triggered when the events occur.
