Enable allowNamespaces in transform-typescript by default

[JLHwung]

Q	A
Fixed Issues?	https://github.com/babel/notes/blob/master/2019/05/21.md#prs
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?	Y
Tests Added + Pass?	Yes
Documentation PR Link	babel/website#2449
Any Dependency Changes?
License	MIT

Unlike useSpread in preset-react which will be materialized in Babel 8, I don't think the TSNamespace feature is ready for that, but we can enable it by default in Babel 8 and gather more feedback.

Note that allowNamespaces has been enabled by default in preset-typescript (#12460), since it was extracted from babel-8-dev.

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Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/39897/

[nicolo-ribaudo]

Is this actually a breaking change, or can we do it in a minor?

[JLHwung]

Agreed that it can be shipped in a minor, since we are not removing this option.

[nicolo-ribaudo]

Imho we are ready to enable this by default now (in 7.13.0).

We introduced namespace support in 7.5.0 (July 2019) behind a flag because it was a big feature and we wanted people to test it as if it was a "beta".

After more than one year we got a few bug reports and we fixed them, and probably they are now as stable as any other Babel feature.