[css-mixins-1] Split CSS Mixins Level 1 into levels

[fantasai]

The https://drafts.csswg.org/css-mixins-1/ ED defines three very distinct features at three very distinct levels of stability. I think it would be helpful to split it into three levels, to communicate better where things are at and to aid in wide review.

1. Functions
2. Mixins
3. Macros

[css-meeting-bot]

The CSS Working Group just discussed [css-mixins-1] Split CSS Mixins Level 1 into levels, and agreed to the following:

• RESOLVED: Cut a new version of mixins, leave functions in level 1

The full IRC log of that discussion

<Kurt> scribe+
<Kurt> fantasai clear from discussions that mixins is not stable, but functions is shipping
<Kurt> ...are sticky features. I suggested we split into two levels and move functions to CR
<Kurt> ...whereas mixins is still exploratory, different levels of stability, and mixing them up is problematic
<Kurt> tabatkis: OK with this
<Kurt> TabAtkins: OK with this
<lea> +1
<miriam> works for me
<Kurt> PROPOSED RESOLUTION: Cut a new version of mixins, leave functions in level 1
<Kurt> RESOLVED: Cut a new version of mixins, leave functions in level 1