syntax: Highlight Goal in any context

[Lysxia]

Reported by Thomas Lamiaux on Zulip. Goal is followed directly by the proposition to prove, which can be any term so, unlike Theorem etc., it's practically impossible to use lookarounds to disambiguate the Goal command from an identifier.

Goal prop.
(* different from *)
Theorem ident : prop.

[rtetley]

Just took the liberty of rebasing and will merge after CI