fix(confirm) --timeout was ignored, now works as documented

[dpritchett]

Fixes #698.

Relevant excerpts from gum confirm --help

Usage: gum confirm [<prompt>] [flags]

Ask a user to confirm an action

Arguments:
  [<prompt>]    Prompt to display.

Flags:
     --default              Default confirmation action
     --timeout=0            Timeout until confirm returns selected value or default if provided ($GUM_CONFIRM_TIMEOUT)

Changes

• Chains a .WithTimeout(o.Timeout) in the confirm command's huh.NewForm() chain under confirm.Run()
• Adds an errIsValidTimeout(error) bool method to confirm.Options to facilitate safely ignoring the timeout error returned in the course of the Run() context expiring due to timeout
• Adds unit tests for errIsValidTimeout

Demos

Before: doesn't time out without outside help

Note that five seconds elapse here before timeout kills gum, returning a nonzero exit code.

Set TypingSpeed 0.1
Output broken-main.gif
Set Shell zsh
Sleep 500ms
Type "git checkout main && git fetch upstream main && git rev-parse HEAD"
Enter
Type "date +%H:%M:%S.%1N && go run . confirm --timeout=1s --default='Yes' && echo 'Success'"
Enter
Sleep 4s
Ctrl+C
Type "date +%H:%M:%S.%1N"
Enter
Sleep 4s
Ctrl+D

After: times out as expected

Times out in ~1 second as desired.

Output fixed.gif
Set Shell zsh
Sleep 500ms
Type "git checkout bug-confirm-timeout-was-missing && git rev-parse HEAD"
Enter
Sleep 1.5s
Type "go run . confirm --timeout=1s 'Am I going to time out?' --default='Yes' && echo Success"
Enter
Sleep 3s
Ctrl+D

Also works with GUM_CONFIRM_TIMEOUT.