Check for non-t-shirt size values

[taysea]

Please describe what the rule should do:

Flag if someone has provided a non-t-shirt size for a margin, pad, gap, width, etc. value. Shouldn't be enabled by default but should be opted-into with HPE Design System.

What category of rule is this? (place an "X" next to just one item)

[x] Warns about a potential error (problem)
[ ] Suggests an alternate way of doing something (suggestion)
[ ] Other (please specify:)

Provide 2-3 code examples that this rule will warn about:

<Box width="300px" />
<Box pad="22px" />

Why should this rule be included in eslint-plugin-grommet?

Best practice for HPE Design System.