Introduce JSON::Coder #786

Job	Run time
Ruby memcheck	53s
ruby-versions / Generate Ruby versions	3s
ubuntu-latest 2.7	57s
ubuntu-latest 3.0	35s
ubuntu-latest 3.1	36s
ubuntu-latest 3.2	34s
ubuntu-latest 3.3	46s
ubuntu-latest 3.4	40s
ubuntu-latest head	1m 26s
ubuntu-latest jruby	1m 9s
ubuntu-latest jruby-head	2m 28s
macos-14 2.7	48s
macos-14 3.0	19s
macos-14 3.1	31s
macos-14 3.2	24s
macos-14 3.3	23s
macos-14 3.4	28s
macos-14 head	21s
macos-14 jruby	1m 1s
macos-14 jruby-head	44s
windows-latest 2.7	3m 57s
windows-latest 3.0	2m 26s
windows-latest 3.1	2m 32s
windows-latest 3.2	1m 53s
windows-latest 3.3	3m 18s
windows-latest 3.4	4m 3s
windows-latest head	2m 4s
ubuntu-24.04-arm 3.4	44s
macos-13 3.4	36s
windows-latest mswin	2m 27s
ubuntu-latest jruby-9.4	1m 1s
macos-latest truffleruby-head	1m 5s
ubuntu-latest truffleruby-head	2m 58s
	44m 10s

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