[scroll-animations-1] Require <dashed-ident> for timeline names

Split out from https://github.com/w3c/csswg-drafts/issues/7759:

@tabatkins wrote:

> timeline names are `<custom-ident>` right now. Is that a necessity? Making it `<dashed-ident>` would be nice since it would avoid the grammar ever clashing in the future with new keywords.

+1, this will make future changes to the syntax easier.