[feat] Create `<rh-icon>` element

[markcaron]

With RH Icons updated, we need to get <rh-icon> out the door as soon as possible.

I'm assuming the underlying code will be very much like PF Icon if not identical, but using RH Icon SVGs.

For example:

<rh-icon icon="acorn" icon-set="standard">

Sets

1. Standard (Default)
2. UI
3. Micron
4. Social

[marionnegp]

@markcaron @bennypowers, according to the Brand site's info about icons, icons shouldn't be any bigger than 100px. Our largest icon size token is 128px. Should we remove that icon size and have our largest icon size token be 96px?

[marionnegp]

Updated icon set sizes from Brand:

• Standard icons = 24px - 100px (preferably 36px+)
• UI icons = 14px - 24px
• Microns = 8px - 12px

They don't have specific guidelines for social icons, but they should generally be used like UI icons.

[marionnegp]

@coreyvickery @markcaron, could you take a look at the docs for <rh-icon>? I added a couple comments that the code on the Accessibility subpage could change after development is done.