Support for `rb_io_mode_t`. #392

Job	Run time
ruby-versions / Generate Ruby versions	3s
windows-latest 2.7	3m 19s
windows-latest 3.0	1m 22s
windows-latest 3.1	1m 7s
windows-latest 3.2	1m 48s
windows-latest 3.3	2m 25s
windows-latest 3.4	2m 25s
windows-latest jruby-head	2m 5s
windows-latest mingw	1m 4s
windows-latest mswin	59s
	16m 37s

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