precompilation: prevent accidental or pasted keystroke triggers during monitoring

[IanButterworth]

Fixes #61520

@topolarity from some quick testing this fixes the issue for me. Can you try it out?
If I paste in

(@v1.14) pkg> add Example
julia> using Example

I get

(@v1.14) pkg> add Example
    Updating registry at `~/.julia/registries/General.toml`
   Resolving package versions...
    Updating `~/.julia/environments/v1.14/Project.toml`
  [7876af07] + Example v0.5.5
    Updating `~/.julia/environments/v1.14/Manifest.toml`
  [7876af07] + Example v0.5.5
Precompiling project...
Precompiling Example finished.
  1 dependency successfully precompiled in 1 seconds

julia> using Example

i.e. none of the pasted chars are consumed by the monitor

And pressing i now requires confirmation, like c.

Developed with claude:

---

Drain any pre-existing input from stdin (e.g. pasted or pre-typed text) before entering the precompilation key listener, and replay it back into stdin on exit so it is not lost.

Additionally, the i key (send profiling signal) now requires Enter to confirm, matching the existing c (cancel) behavior. This prevents a single accidental keypress from triggering a noisy profiling dump.

Internally, the separate cancel_confirming/cancel_confirm_deadline fields are replaced with a general confirming::Symbol and single confirm_deadline, making it easy to add confirmable actions.