🖼 Add icon node

[agoose77]

This PR adds support for a subset of the icon syntax from sphinx-design. Although this new role supports the legacy role names such as {fab}, the preferred aliases are:

'icon:oct',
'icon:mrg',
'icon:mol',
'icon:mrd',
'icon:msp',
'icon:mtt',
'icon:fab',
'icon:far',
'icon:fas'

[changeset-bot]

🦋 Changeset detected

Latest commit: 58ef048

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
myst-ext-icon	Minor
myst-cli	Patch
mystmd	Patch

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[agoose77]

@rowanc1 should we look to merging this ahead of getting theme support? Do you concur that it's worth targeting the same icon packs as sphinx-design?

[rowanc1]

Let's talk about the icon packs we support when we build out the theme (probably lower priority than some of the other things we have on at the moment).