No Heading level 1 inside a Layer

[taysea]

Please describe what the rule should do:

Should mark if Heading Level 1 is used inside a Layer and should suggest using Heading level 2 instead.

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:

<Layer>
   <Heading level={1}>Layer title</Heading>
</Layer>

<Layer>
   <Heading>Layer 2 title</Heading> // level 1 is default
</Layer>

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

Accessibility best practices.