Strict mode

[eevee]

I call it strict mode, but it should really be the default, and should only be turned off for backwards compatibility.

Some things that should be errors:

• Invalid syntax
• @include with an unrecognized mixin name
• Unrecognized variable names

Note that some particularly egregious constructs like IE's expression still need to make it through unscathed.

Some things that are hard to make strict, but may be opt-in warnings or linters:

• Unrecognized function name (i.e. not in any known CSS either)
• Relying on -/_ equivalence