What does this PR do?

update searchPlaceholder to take a node as well as a string

Where should the reviewer start?

select/index.d.ts

What testing has been done on this PR?

How should this be manually tested?

searchPlaceholder functionally takes a node as well as a string so should we support this?

Do Jest tests follow these best practices?

• screen is used for querying.
• The correct query is used. (Refer to this list of queries)
• userEvent is used in place of fireEvent.
• asFragment() is used for snapshot testing.

Any background context you want to provide?

I want search icon and placeholder text in search input field.
But searchPlaceholder accept only string value and I want both search Icon and placeholder both like below screenshot
How to do that? (edited) 

https://codesandbox.io/s/competent-merkle-q908cc?file=/src/App.js

What are the relevant issues?

none

Screenshots (if appropriate)

Do the grommet docs need to be updated?

yes

Should this PR be mentioned in the release notes?

maybe

Is this change backwards compatible or is it a breaking change?

backwards compatible