bug: loss of input on ReleaseTerminal / Bubbletea shutdown

It is not currently possible to integrate Bubbletea-based programs in scripts or unit tests that buffer terminal input
across multiple runs of the Bubbletea event loop.

The specifics are explained here: https://dr-knz.net/bubbletea-control-inversion.html

In summary, the input reader function is "greedy" but drops/forgets input event messages during `ReleaseTerminal()`. This is where the input is lost.

The text linked above outlines a solution:

- [x] invert control of the input reader function, i.e. this PR: https://github.com/charmbracelet/bubbletea/pull/569
- [ ] synchronise the reading from the external input with `Update` calls.
- [ ] for the benefit of programs (like Bubbline) that need to share control of the terminal, introduce a new API to suspend the Bubbletea event loop.