Convert to extension types

[mit-mit]

Convert to use extension types; what was formerly known as inline classes.

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[srujzs]

Up to you on how you want to write these, but I've been writing the primary constructor as HttpClient._(JSObject obj) when we only ever use the external constructor so that we don't need a named external constructor.

[mit-mit]

Done

[devoncarew]

One comment on the changelog entry.

Looking forward to extension types! Seems like they'll be very handy for a number of use cases.

[devoncarew]

The previous entry hasn't been published, so we can just combine this entry with that one (we don't need a new v1.5.2-dev section).

[mit-mit]

Done