Add IO.foreach and IO.readlines on Node.js - #2807

Fixes #1845.

What changed

IO.foreach reads a file line by line without materialising the whole thing as an array, and is the form most Ruby code reaches for. It did not exist on Node.js, so it raised NoMethodError: undefined method 'foreach' for IO. The class-level IO.readlines was missing for the same reason, so both are added here.

Both delegate to File.open's block form, which closes the handle, and to the existing IO#each_line and IO#readlines. The separator handling and the enumerator behaviour come from code that is already tested rather than from a second implementation, which is why the diff is 16 lines.

IO#each_line landed in #2309, which is what made this a thin delegation rather than real work.

Behaviour checked against MRI:

File.foreach inherits both, since MRI defines them on IO's singleton and File subclasses it. Verified rather than assumed.

How to test

printf 'a\nb\nc\n' > /tmp/f.txt
bundle exec opal -rnodejs -e "IO.foreach('/tmp/f.txt') { |l| p l }"

Verification I ran locally:

• bundle exec rake minitest_node_nodejs — 87 runs, 160 assertions, 0 failures
• bundle exec rake mspec_nodejs — 14626 examples, 0 failures
• bundle exec rubocop — 226 files, no offenses

Seven tests added to test/nodejs/test_io.rb, covering each row of the table above. Reverting just stdlib/nodejs/io.rb and rerunning turns them red with NoMethodError: undefined method 'foreach' for IO, so they pin the feature rather than the implementation.

Note for anyone rerunning these: test/nodejs/ is driven by minitest_node_nodejs. The similarly named minitest_cruby_nodejs runs the CRuby suite instead and passes whether or not this change is present.

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