Introduce JSON::Coder #691

Job	Run time
Ruby memcheck	1m 1s
ruby-versions / Generate Ruby versions	3s
ubuntu-22.04 2.7	54s
ubuntu-22.04 3.0	31s
ubuntu-22.04 3.1	33s
ubuntu-22.04 3.2	32s
ubuntu-22.04 3.3	29s
ubuntu-22.04 head	1m 16s
ubuntu-22.04 jruby	47s
ubuntu-22.04 jruby-head	52s
macos-13 2.7	2m 15s
macos-13 3.0	46s
macos-13 3.1	42s
macos-13 3.2	36s
macos-13 3.3	41s
macos-13 head	32s
macos-13 jruby	1m 14s
macos-13 jruby-head	1m 25s
macos-14 2.7	57s
macos-14 3.0	23s
macos-14 3.1	33s
macos-14 3.2	23s
macos-14 3.3	19s
macos-14 head	18s
macos-14 jruby	31s
macos-14 jruby-head	30s
windows-latest 2.7	5m 19s
windows-latest 3.0	2m 22s
windows-latest 3.1	2m 23s
windows-latest 3.2	2m 58s
windows-latest 3.3	2m 36s
windows-latest head	1m 26s
windows-latest mswin	2m 25s
ubuntu-latest jruby-9.4	50s
macos-latest truffleruby-head	44s
ubuntu-latest truffleruby-head	2m 4s
	42m 10s

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