fix: measure does not measure when an exception occurs

[GabrielLidenor]

closes #533

Why

The failure occurred because an exception raised by result = block.() stopped normal execution before the elapsed time could be calculated and displayed.

Wrapping the evaluation in begin...ensure guarantees that the timing code runs even when the evaluated block raises an exception. The original exception is still propagated and handled normally afterward.

Proof

 irb git:(fix-measure-in-exception) bundle exec ruby -Ilib exe/irb

⢀⡴⠊⢉⡟⢿  IRB v1.18.0 - Ruby 4.0.1
⣎⣀⣴⡋⡟⣻  "show_doc method" to view documentation
⣟⣼⣱⣽⣟⣾  ~/Projects/irb

irb(main):001> measure
TIME is added.
irb(main):002> Regexp.compile('^a*b?a*()\1$', timeout: 10.0) =~ "a" * 50000 + "x"
processing time: 10.000346s
(irb):2:in '<main>': regexp match timeout (Regexp::TimeoutError)
	from exe/irb:9:in '<main>'
irb(main):003>

[tompng]

Looks good. Thank you!

Just one note:
Calling Kernel.exit will also print processing time, but it won't be a problem.

irb(main):001> measure
TIME is added.
irb(main):002> Kernel.exit
processing time: 0.000079s

IRB's exit command won't print it.