Prevent loading both `json/ext` and `json/pure` #126

Job	Run time
Ruby memcheck	50s
ruby-versions / Generate Ruby versions	3s
ubuntu-22.04 2.7	1m 10s
ubuntu-22.04 3.0	33s
ubuntu-22.04 3.1	34s
ubuntu-22.04 3.2	29s
ubuntu-22.04 3.3	32s
ubuntu-22.04 head	1m 24s
ubuntu-22.04 jruby	1m 8s
ubuntu-22.04 jruby-head	1m 8s
macos-13 2.7	3m 3s
macos-13 3.0	36s
macos-13 3.1	42s
macos-13 3.2	42s
macos-13 3.3	52s
macos-13 head	39s
macos-13 jruby	1m 42s
macos-13 jruby-head	2m 8s
macos-14 2.7	1m 5s
macos-14 3.0	23s
macos-14 3.1	31s
macos-14 3.2	35s
macos-14 3.3	22s
macos-14 head	17s
macos-14 jruby	52s
macos-14 jruby-head	44s
windows-latest 2.7	4m 27s
windows-latest 3.0	2m 14s
windows-latest 3.1	2m 19s
windows-latest 3.2	2m 39s
windows-latest 3.3	3m 12s
windows-latest head	1m 25s
windows-latest mswin	1m 54s
ubuntu-latest jruby-9.4	1m 10s
macos-latest truffleruby-head	41s
ubuntu-latest truffleruby-head	1m 50s
	44m 55s

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