Change enum syntax to reflect real constraints

[martinthomson]

What is the issue with the Web IDL Standard?

@marcoscaceres noted that there are constraints on implementations that mean that enums can't contain certain characters. For instance, "." caused problems in WebKit because the string is converted to a C++ constant name. I imagine that "foo" and "FOO" together in the same enum might also lead to bad outcomes, as would "foo-bar" and "foo_bar".

This probably hasn't caused a problem so far because people do sensible things. Task here is to work out what the real constraints are and document them.

[annevk]

As long as https://w3ctag.github.io/design-principles/#casing-rules is followed it should all be fine. We could maybe point that out or we could try to go even further and enforce it to some extent, but there's probably some exceptions out there.

However, I don't think we need to look at what actual implementations try to enforce. If we decide that the web platform needs "foo.bar" implementations have a variety of ways to make that possible and that's really up to them.