Support for `rb_io_mode_t`. #393

Job	Run time
ruby-versions / Generate Ruby versions	2s
windows-latest 2.7	2m 45s
windows-latest 3.0	1m 19s
windows-latest 3.1	2m 19s
windows-latest 3.2	1m 42s
windows-latest 3.3	1m 26s
windows-latest 3.4	1m 51s
windows-latest jruby-head	1m 48s
windows-latest mingw	1m 14s
windows-latest mswin	1m 15s
	15m 41s

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