add SingleSelectElement component

[davemeyer]

add a new component for the single select slack block our existing SelectElement was a multi-select component for the multi select slack block

so rename what was SelectElement to MultiSelectElement and the new single select is called SingleSelectElement so we are explicit about the use cases

closes: #47

[davemeyer]

I think you might have forgotten to add to exports in index. ts actually

@andycmaj i put the single and multi select in the same file. but can update for consistent 1:1 mapping. but as-is, we do a export * from './SelectElement'; so it is already exported

[andycmaj]

right on. thanks!