[css-variables] [css-syntax] Define how Unicode name comparisons work

[litherum]

Because names support all of Unicode, and various places need to compare these for equality (e.g. comparing a custom property name to the first argument of var()) the spec should mention how these equality operations work.

Unicode equality comparisons generally involve the Unicode Collation Algorithm. This incorporates the Unicode Normalization routines, which may be sufficient.

An easier-to-compute option is to say that these names are compared byte-by-byte.

[tabatkins]

In Variables§2, custom property names are defined to be case-sensitive, which means they're compared code-point by code-point.

Values§3.2 also mentions this as a general rule for <custom-ident>s, but doesn't link case-sensitive properly; I'll fix that.

The rest of CSS is limited to ASCII and is ASCII case-insensitive.