[css-values-5] Invalidation of ident()

[bramus]

This is a follow-up to #9141 which introduces ident(). Something that is not defined yet is what should happen when the input for ident() is invalid.

E.g. what happens if you do the following:

@property --length {
  syntax: "<length>";
  inherits: false;
  initial-value: 0px;
}

.target {
  --length: 120px;
  anchor-name: ident("--anchor-" var(--length));
}

Because --length is a <length>, ident() can’t generate a valid <custom-ident>. I assume that in this case the declaration should become Invalid At Computed Value Time (IACVT) and the value should be considered to be unset.

Additionally, we might consider adding an extra argument to ident(), to allow a fallback value in case the constructed value is not a valid <custom-ident> – this similar to var().

The definition for ident() would then become the following:

<ident-fn> = ident(<ident-args>, <declaration-value>?);
<ident-args> = [<string> | <integer> | <ident>]+