Prevent loading both `json/ext` and `json/pure` #128

Job	Run time
Ruby memcheck	48s
ruby-versions / Generate Ruby versions	7s
ubuntu-22.04 2.7	1m 8s
ubuntu-22.04 3.0	30s
ubuntu-22.04 3.1	39s
ubuntu-22.04 3.2	29s
ubuntu-22.04 3.3	41s
ubuntu-22.04 head	1m 23s
ubuntu-22.04 jruby	1m 13s
ubuntu-22.04 jruby-head	1m 12s
macos-13 2.7	2m 16s
macos-13 3.0	43s
macos-13 3.1	46s
macos-13 3.2	45s
macos-13 3.3	43s
macos-13 head	38s
macos-13 jruby	1m 15s
macos-13 jruby-head	2m 34s
macos-14 2.7	1m 5s
macos-14 3.0	23s
macos-14 3.1	26s
macos-14 3.2	21s
macos-14 3.3	28s
macos-14 head	23s
macos-14 jruby	42s
macos-14 jruby-head	53s
windows-latest 2.7	4m 38s
windows-latest 3.0	2m 17s
windows-latest 3.1	2m 30s
windows-latest 3.2	3m 30s
windows-latest 3.3	2m 27s
windows-latest head	1m 26s
windows-latest mswin	1m 52s
ubuntu-latest jruby-9.4	1m 13s
macos-latest truffleruby-head	48s
ubuntu-latest truffleruby-head	2m 4s
	45m 16s

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