Rename `f_optarg` to `f_opt_arg` for consistency in parser rules #645

Job	Run time
make (check, cppflags=-DVM_CHECK_MODE)	7m 33s
make (test-bundled-gems)	17m 26s
make (check, ubuntu-24.04)	7m 54s
make (check, i686)	9m 20s
make (test-bundler-parallel, 50)	25m 57s
make (check, --enable-shared --enable-load-relative)	7m 12s
make (check, ubuntu-24.04-arm)	7m 34s
make (check, --disable-yjit)	7m 52s
Ubuntu result	3s
	1h 30m 51s

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