Flag for non t-shirt size values

[taysea]

Please describe what the rule should do:

Flags for border, radius, pad, margin, gap, height, width, size values that aren't t-shirt sizes from theme.

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

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

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

<Box width="100px" ... />
<Text size="24px" />
<Box width="50%" ... />
<Button margin={{ top: '50px' }} />

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