Support double click recognition in `bevy_picking`.

## What problem does this solve or what need does it fill?

`bevy_picking` currently only able to recognize the type of clicks, but can't, for counts.

## What solution would you like?

Add a `DoubleClick` type.

## What alternative(s) have you considered?

Supporting `MultipleClick` would be better.

## Additional context

None

Start by reading the click-recognition implementation in bevy_picking and trace how existing click types are represented and detected. Compare the requested DoubleClick behavior with the suggested generic MultipleClick alternative, then verify the chosen API recognizes double clicks without breaking existing click handling.

Written by the indexing model from the issue text.