Add an option to prefill the search field of select

[schrej]

This PR adds an option to prefill the search field of the select prompt.

The cursor will use it as it's initial value, which causes it to be prefilled on the first switch to search mode (either by setting StartInSearchMode or by pressing /).

My use case is a cli tool that allows to prefil the value with a parameter.

[CLAassistant]

All committers have signed the CLA.