Files field: upload keyboard shortcuts

Original idea: https://kirby.nolt.io/173

Adding key shortcuts to either add an existing or upload a new file, like ctlr+click for the one and shift+click for the other.

I would also vote for implementing general keyboard shortcuts that everyone knows and uses on a daily basis, like https://support.microsoft.com/en-us/windows/keyboard-shortcuts-in-windows-dcc61a57-8ff0-cffe-9796-cb9706c75eec