Introduce JSON::Coder #778

Job	Run time
Ruby memcheck	57s
ruby-versions / Generate Ruby versions	3s
ubuntu-latest 2.7	1m 5s
ubuntu-latest 3.0	34s
ubuntu-latest 3.1	36s
ubuntu-latest 3.2	33s
ubuntu-latest 3.3	1m 28s
ubuntu-latest 3.4	35s
ubuntu-latest head	1m 33s
ubuntu-latest jruby	1m 6s
ubuntu-latest jruby-head	1m 6s
macos-14 2.7	50s
macos-14 3.0	27s
macos-14 3.1	24s
macos-14 3.2	20s
macos-14 3.3	27s
macos-14 3.4	23s
macos-14 head	19s
macos-14 jruby	36s
macos-14 jruby-head	44s
windows-latest 2.7	4m 26s
windows-latest 3.0	2m 46s
windows-latest 3.1	3m 37s
windows-latest 3.2	2m 22s
windows-latest 3.3	3m 25s
windows-latest 3.4	2m 47s
windows-latest head	1m 32s
ubuntu-24.04-arm 3.4	44s
macos-13 3.4	36s
windows-latest mswin	1m 15s
ubuntu-latest jruby-9.4	1m 5s
macos-latest truffleruby-head	44s
ubuntu-latest truffleruby-head	2m 8s
	41m 33s

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