[css-mixins] Add CSS Functions and Mixins specification (#10367)

* [css-mixins] Add CSS Functions and Mixins specification

To begin with, only @function is described.

* bikeshed-clean

* Generate missing Big Text.

* "An at-function" -> "A function"

There's apparently no right answer to "an at-foo" vs "a foo",
but we can at least stay consistent within the same spec.

* Resolve to the default value when the type is wrong.

* Update css-mixins-1/Overview.bs

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Co-authored-by: Tab Atkins Jr <[email protected]>
Co-authored-by: Miriam Suzanne <[email protected]>