Allow free form text input in Combobox

[regdocs]

This is useful when the options list is very large (e.g., repos with 2000+ branches), where rendering the full popover list becomes expensive.

An alternative would be a virtual list.

Ref:
https://github.com/frappe/press/blob/develop/dashboard/patches/frappe-ui%2B0.1.238.patch

Implemented as a patch to frappe-ui v0.1.238 in press.