Move shared types and other primitives to a standalone module

[jannyHou]

Description

Chatted with @raymondfeng

context see #580 (comment)

Some files under /repository like mixin.ts, common-types.ts could be organized to /lang to export language specific library functions.

From Raymond

I think some of them should be refactored into a common module such as @loopback/common
or @loopback/lang
There are a few things related to language
such as Constructor, Mixin etc
We’ll have @loopback/model and/or @loopback/types etc

loopback/lang sounds more straightforward to me.

[bajtos]

IMO, MixinBuilder in particular should not be something to widely recommend to our users, we should be telling them to use mixins directly via ES2015/TS syntax. See loopbackio/loopback.io#491 (comment) and loopbackio/loopback.io#491 (comment) for more context.

[bajtos]

As for package name, I vote for @loopback/core-types.

[raymondfeng]

I vote for @loopback/common-types or @loopback/common

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