Update the mixin docs

[orta]

Updates the mixin docs based on microsoft/TypeScript#39118

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📖	Deployed to a PR branch - playground tsconfig old handbook

Got false positives?

Make changes to the global settings spellcheck.json in /artsy/peril-settings.

Typos for Mixins.md

Line	Typo
9	be familiar with the idea of mixins or traits for languages like
11	## How Does A Mixin Work?
14	TypeScript's best mixin support is done via the class
17	d a class which will have the mixin's applied on top of:
60	epresents the base class with mixins applied:
98	## Constrained Mixins
100	In the above form, the mixin's have no underlying knowledge
131	Then you can create mixins which only work when you have
163	nt recommended a way to write mixins where you created both the ru
211	The mixin pattern is supported natively
214	#### Decorators and Mixins [#4881](https://github.com/
216	not use decorators to provide mixins via code flow analysis:
246	#### Static Property Mixins [#17829](https://github.com

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[orta]

This got peer reviewed at the docs meeting