Support for `rb_io_mode_t`. #390

Job	Run time
ruby-versions / Generate Ruby versions	2s
windows-latest 2.7	3m 10s
windows-latest 3.0	1m 19s
windows-latest 3.1	2m 33s
windows-latest 3.2	2m 25s
windows-latest 3.3	3m 44s
windows-latest 3.4	2m 39s
windows-latest jruby-head	1m 54s
windows-latest mingw	1m 44s
windows-latest mswin	2m 43s
	22m 13s

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