Make `BaseGroovyExtension` and `BaseKotlinExtension` public #1053

Job	Run time
spotlessCheck assemble testClasses	49s
build (maven, 11, ubuntu-latest)	35s
build (maven, 17, ubuntu-latest)	33s
build (gradle, 11, ubuntu-latest)	4m 30s
build (gradle, 17, ubuntu-latest)	6m 26s
build (maven, 11, windows-latest)	1m 45s
build (gradle, 17, windows-latest)	9m 4s
build (npm, 11, ubuntu-latest)	48s
	24m 30s

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