Hotfixes: Update SelectMenu.Item type & extend Box in Position

[emplums]

A couple of hotfixes after reviewing the 19.0.0 release:

• Make the as prop on SelectMenu.Item optional
• I wanted to update the SelectMenu.Modal code example to make sure folks know that the component needs to be wrapped in a relatively positioned element for the align prop to work. Which made me realize the the Position family of components should have all the same props as Box but they don't because we weren't extended Box so I fixed that.

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[emplums]

I removed all of these because they are already included in the definition for StyledSystem.PositionProps

[emplums]

I removed BaseProps because the two components that extend PositionProps (Position & Popover) already are including BaseProps in their definition

[emplums]

these are all included in BoxProps

[emplums]

@BinaryMuse if you could review this again, I've added one more change.

I also think we should consider putting FLEX in LAYOUT in our constants file, to insure that anywhere we allow folks to use the layout props they also get the flex props. I started working on that in this branch but it got a lil hairy and I think would be fine as a follow-up after this release. It should be an internal API change with no breaking changes.

[BinaryMuse]

One thing I forgot — we should update the Position docs

[emplums]

@BinaryMuse good catch! 💯 more points for automating this eventually 😂

[BinaryMuse]

Sweeeeet