Add Number component

[IamPete1]

This adds a number component with optional min/max/step and units.

<sdpi-item label="Number">
    <sdpi-number setting="numberSetting" min="1" max="5" step="1" default="3" units="Px"></sdpi-number>
</sdpi-item>

closes https://github.com/GeekyEggo/sdpi-components/issues/18