Use IO's encoding instead of Encoding.default_external

[monkeyWzr]

Fix #755. Use IO object's encoding to separate from global Encoding.default_external.

For example:

require 'reline'
::Encoding.default_external = ::Encoding::ASCII_8BIT
::Encoding.default_internal = ::Encoding::ASCII_8BIT

input = STDIN.dup
input.set_encoding(Encoding::UTF_8)
Reline.input = input

ascii_text = Reline.readline('prompt > ', false)

$stderr.puts ascii_text.encoding.to_s

and get UTF-8:

prompt > こんにちは
UTF-8

[monkeyWzr]

This change fails one of irb's test. Should be fixed by ruby/irb#1022

[tompng]

The parameter encoding: can be also removed

[monkeyWzr]

💦 Thanks! Removed!

[monkeyWzr]

FYI, one of irb's integration test seems to randomly fail.

https://github.com/ruby/reline/actions/runs/11424348590/job/31784575039

===============================================================================
Failure: test_ctrl_c_is_handled(IRB::RenderingTest::TestVTerm)
/home/runner/.rubies/ruby-head/lib/ruby/gems/3.4.0+0/bundler/gems/yamatanooroti-3937fa311781/lib/yamatanooroti/vterm.rb:155:in 'block in Yamatanooroti::VTermTestCaseModule#assert_screen'
/home/runner/.rubies/ruby-head/lib/ruby/gems/3.4.0+0/bundler/gems/yamatanooroti-3937fa311781/lib/yamatanooroti/vterm.rb:141:in 'Yamatanooroti::VTermTestCaseModule#retryable_screen_assertion_with_proc'
/home/runner/.rubies/ruby-head/lib/ruby/gems/3.4.0+0/bundler/gems/yamatanooroti-3937fa311781/lib/yamatanooroti/vterm.rb:153:in 'Yamatanooroti::VTermTestCaseModule#assert_screen'
/home/runner/work/reline/reline/irb/test/irb/yamatanooroti/test_rendering.rb:316:in 'IRB::RenderingTest::TestVTerm#test_ctrl_c_is_handled'
<"irb(main):001>\n" + "^C\n" + "irb(main):001>\n"> expected but was
<"irb(main):001> ^C\n" + "irb(main):001>\n">

diff:
+ irb(main):001> ^C
  irb(main):001>
- ^C
- irb(main):001>
===============================================================================

[tompng]

randomly fail.

I think it is not related to this change.
Maybe it is because Reline prints prompt before set_signal_handlers. IRB's test is sending ctrl-c too early (need a sleep in test), or Reline can print prompt after set_signal_handlers.
I'll check it again if it fails often.

[tompng]

Looks good. Thank you 👍
STDIN.gets returns string with STDIN's encoding. Reline now aligns with it.