Move the jupytext-config helpers to a subpackage, and add tests #84

Job	Run time
pre-commit / pre-commit	17s
pre-commit / lint-extension	1m 57s
codeql / codeql	3m 9s
test-pip / test-pip (3.8, ~=3.0)	9m 8s
test-pip / test-pip (3.9, ~=3.0)	8m 57s
test-conda / test-conda (ubuntu-latest, 3.11)	10m 45s
test-pip / test-pip (3.10, ~=3.0)	9m 3s
test-conda / test-conda (macos-latest, 3.11)	11m 6s
test-ui / test-ui	4m 34s
test-pip / test-pip (3.11, ~=3.0)	8m 56s
test-unit-functional-integration / test-pip (3.11, unit)	2m 27s
test-unit-functional-integration / test-pip (3.11, functional)	3m 34s
test-pip / test-pip (3.12-dev, ~=3.0)	9m 12s
test-unit-functional-integration / test-pip (3.11, integration)	2m 57s
test-unit-functional-integration / test-pip (3.11, external)	5m 46s
test-pip / test-pip (3.x, ~=2.0)	9m 13s
test-pip / test-pip (3.x)	9m 13s
test-pip / test-pip (3.x, true)	9m 14s
test-conda / test-conda (windows-latest, 3.11)	10m 57s
test-pip / test-pip (3.x, true)	9m 13s
build	0s
	2h 19m 38s

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