Use a separate Jinja env per extension #818

Job	Run time
test-js (ubuntu-latest)	28s
test-js (macos-latest)	31s
test-js (windows-latest)	51s
test-py (ubuntu-latest, 3.11, py)	1m 6s
test-py (ubuntu-latest, 3.11, old_deps)	49s
test-py (ubuntu-latest, 3.8, py)	1m 7s
test-py (ubuntu-latest, 3.8, old_deps)	58s
test-py (macos-latest, 3.11, py)	1m 22s
test-py (macos-latest, 3.8, py)	1m 15s
test-py (windows-latest, 3.11, py)	1m 38s
test-py (windows-latest, 3.8, py)	1m 42s
build-pyinstaller	1m 29s
build-docs	49s
lint-js	1m 16s
lint-python	1m 20s
	16m 41s

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