Revise allowed characters in identifiers

[tomek-he-him]

• Disallow uppercase characters.

Why: HTML5 is all lowercase. <parametric-svg A="5"> is interpreted as <parametric-svg a="5"> – so it’s impossible to pass the value for A this way. HTML5 is our primary target – so it can’t stay this way.

• Allow non-ASCII chars.

Why: It’s common in maths to use characters like π.

[tomek-he-him]

Disallow uppercase characters.

We might as well say identifiers are case-insensitive. That would be more consistent with HTML5 – and familiar for Windows folks.