Adjust codegen to allow custom `mainargs.TokensReader`s and exercise it via example tests for `Bundle Libraries` docs #93

Job	Run time
test-docs	10m 15s
build-windows / run	4m 56s
build-linux / run	3m 17s
lint-autofix / run	4m 9s
compiler-bridge / run	1m 16s
itest (17, ci/test-mill-bootstrap.sh) / run	8m 56s
linux (11, '{scalajslib,scalanativelib}.__.testCached') / run	15m 30s
linux (11, 'example.{depth,extending}.__.local.testCached') / run	8m 55s
linux (11, 'example.thirdparty[{mockito,acyclic,commons-io}].local.testCached') / run	4m 51s
linux (11, 'integration.{failure,feature,ide}.__.local.testCached') / run	10m 5s
linux (17, '{main,scalalib,testrunner,bsp,testkit}.__.testCached') / run	9m 1s
linux (17, 'example.javalib.__.local.testCached') / run	9m 24s
itest (11, ci/test-mill-dev.sh && ci/test-mill-release.sh && ./mill -i -k __.ivyDepsTree && ./mil... / run	3m 24s
linux (17, 'example.scalalib.__.local.testCached') / run	15m 4s
linux (17, 'example.thirdparty[{fansi,jimfs,netty,gatling}].local.testCached') / run	5m 54s
linux (17, 'integration.invalidation.__.server.testCached') / run	8m 55s
linux (17, 'integration.invalidation.__.fork.testCached') / run	13m 59s
linux (17, contrib.__.testCached) / run	5m 8s
windows (11, "example.scalalib.{basic,web}.__.fork.testCached") / run	8m 29s
windows (11, contrib.__.testCached) / run	8m 27s
windows (11, "{main,scalalib,bsp}.__.testCached") / run	6m 17s
windows (11, 'integration.invalidation[_].server.testCached') / run	6m 20s
windows (17, 'integration.{feature,failure}[_].fork.testCached') / run	6m 28s
	2h 59m 0s

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