Adding new select_prompt method to generate a blank option.

[jwoertink]

Purpose

Fixes #1027

Description

This adds a new select_prompt method to just generate a blank option so it's clear that this is just your prompt.

select_tag(operation.fighters) do
  select_prompt("Choose your character")
  options_for_select(operation.fighters, [{"SubZero", 1}])
end

Checklist

• - An issue already exists detailing the issue/or feature request that this PR fixes
• - All specs are formatted with crystal tool format spec src
• - Inline documentation has been added and/or updated
• - Lucky builds on docker with ./script/setup
• - All builds and specs pass on docker with ./script/test

[paulcsmith]

Perfect! Just one thought on documentation. There are times when you don’t want to show the prompt. For example if the field is required and you’re on the edit page. In that case you can do operation.record to see if you’re working with an existing record. So you could do select_prompt(“whatever”) unless operation.record

I think it’d be cool to document that here or in the guides as an example since it is probably gonna be a common use case and people may not know how to do it.

Code looks good though!