Add fields to bench preset and create seats field

[simonbilskyrollins]

Fixes issue #5367

[quincylvania]

@simonbilskyrollins Thanks! Could you add "minValue": 0 to the seats field? Nobody wants negative seats 😆

Also, consider adding some placeholder like "placeholder": "2, 4, 6..." to the field to indicate that it's numeric.

[simonbilskyrollins]

@quincylvania good point! I've added both of those things to the PR.

[bhousel]

@simonbilskyrollins - thanks for your PR! 👍
@quincylvania - good catch on the "negative seats" 😆